Planctomycetes are Gram negative bacteria diverse in terms of cell biology, ecology and physiology.

Planctomycetes divide by budding and often have appendages such as stalks or holdfasts. The cell walls of planctomycetes do not contain peptidoglycan, like in Gram-positive or Gram-negative bacteria. Instead, these bacteria have proteinaceous cell walls, like in archaea.

The common morphology is often spherical cells roughly 2 μm in diameter, as observed in the species Aquisphaera giovannonii. However, the diversity in cell shape often varies greatly in them. Ovoid and pear-shaped cells have been described in some species, and often occur in rosettes of three to 10 cells.

Bacteria are prokaryotes, lacking well-defined nuclei and membrane-bound organelles, and with chromosomes composed of a single closed DNA circle. They come in many shapes and sizes, from minute spheres, cylinders and spiral threads, to flagellated rods, and filamentous chains.