The Firmicutes are a phylum of bacteria, most of which have the Gram-positive type of cell walls.
The group is typically divided into the Clostridia, which are anaerobic, the Bacilli, which are aerobic, and the Mollicutes, a class of bacteria which do not have cell walls.
On phylogenetic trees, the first two groups show up as paraphyletic or polyphyletic, as do their main genera, Clostridium and Bacillus.
Firmicutes make up the largest portion of the mouse and human gut microbiome. The division Firmicutes as part of the gut flora has been shown to be involved in energy resorption and obesity.