The single most distinguishing feature between slime molds and fungi is how they eat. Slime molds engulf their food by surrounding and absorbing particles whole, the same way an amoeba does. Fungi, by contrast, secrete digestive enzymes into their environment and then absorb the broken-down nutrients through their cell walls. This difference in feeding strategy is so fundamental that it places these two organisms in entirely separate kingdoms of life.
How Slime Molds and Fungi Feed
Slime molds feed through a process called phagocytosis. Their cells physically wrap around bacteria, tiny protozoa, or bits of organic matter and pull them inside to digest. Picture a blob slowly creeping over a log, engulfing microorganisms as it goes. This is essentially what a slime mold does, and it’s the same strategy used by your own white blood cells when they attack an invader.
Fungi take a completely different approach. A fungus releases enzymes outward into whatever it’s growing on, whether that’s a dead tree, a piece of bread, or soil. Those enzymes break down complex organic material externally, and the fungus then absorbs the dissolved nutrients back through its cell walls. Fungi never physically surround or swallow their food. This absorptive feeding method is one reason fungi tend to grow threadlike networks deep into their food source, while slime molds live and move along surfaces.
Different Cell Wall Chemistry
The cell walls of slime molds and fungi are built from entirely different materials. Fungal cell walls contain chitin, the same tough compound found in insect exoskeletons and crustacean shells. Slime mold cell walls contain cellulose, the structural fiber found in plant cells. This chemical difference reflects how distantly related these organisms actually are, despite looking similar to the naked eye.
Body Structure and Movement
Fungi grow as networks of thin, tube-like filaments called hyphae. These filaments branch outward and are typically divided into compartments by internal cross-walls called septa. Nutrients flow through the network, but the fungus itself stays rooted in place. It expands by growing new filaments at the tips, not by physically relocating.
Slime molds have no hyphae at all. The most dramatic example is the plasmodial slime mold, which forms a single enormous cell containing many nuclei. This mass of protoplasm can spread across a surface, pulsing and streaming its internal contents to propel itself toward food or better conditions. A slime mold on your lawn can visibly change position over the course of a day. This ability to move is something no fungus can do.
The internal organization is also strikingly different. In a plasmodial slime mold, all the nuclei within that giant cell divide at exactly the same time in perfect synchrony. Fungal networks, even when they share cytoplasm between cells, can actively block the flow between compartments using specialized protein plugs at their cross-walls. They maintain much more internal compartmentalization than a slime mold’s free-flowing mass.
They Belong to Different Kingdoms
Despite their superficial resemblance, slime molds and fungi are classified in separate kingdoms. Fungi have their own kingdom, Fungi. Slime molds are classified as protists, a broad kingdom that includes many single-celled and simple multicellular organisms. Slime molds are sometimes called “fungus-like protists” because they produce spore-bearing structures that can look remarkably similar to tiny mushrooms or puffballs, which historically caused confusion.
There are roughly 500 known species of slime mold, spread across several distinct groups: plasmodial slime molds (the large blobby ones), cellular slime molds (which aggregate from individual amoeba-like cells), parasitic slime molds, and net slime molds. Despite the shared name, these groups are not all closely related to each other. What they do share is a set of features that clearly separates them from true fungi: cellulose-based cell walls, phagocytic feeding, and amoeba-like mobility during at least part of their life cycle.
Where You’ll Find Them
Fungi tend to extend their filaments deep into whatever they’re decomposing, whether that’s leaf litter, rotting wood, or soil. Slime molds, by contrast, live on surfaces. You’re most likely to spot them on shaded, well-watered mulch or lawns, where they appear as bright yellow, white, or orange blobs creeping across the surface. The classic “dog vomit” slime mold that shows up on garden mulch after rain is a plasmodium, that single giant cell slowly migrating across the surface in search of bacteria to eat.
If you poke a mushroom, it stays put. If you watch a slime mold over several hours, it will have moved. That simple observation captures the most fundamental split between these two organisms: one is anchored and absorbs, the other crawls and engulfs.