Sexual reproduction isogamous, by a fusion of biciliate zoogametes.The genus Gonium represents species closely related to single celled Chlamydomonas and multicellular differentiated Volvox.

Gonium's morphology of colonies of alike cells suggest it is more genetically similar to Chlamydomonas than Volvox, a fact confirmed by phylogenetic analysis.

[4] Gonium represents the first six evolutionary steps of multicellularity;[3] (1) incomplete cytokinesis, (2) partial inversion, (3) rotation of the basal bodies, (4) organismal polarity, (5) transformation of the cell wall into extra-cellular matrix (ECM), (6) genetic control of cell number.

Although the exact order and progression through David Kirk's twelve steps of multicellular evolution [4] are probably not necessarily linear and each occurs more dynamically than originally thought.

The sexual program of Gonium is induced by nitrogen deprivation where each vegetative cell within the colony differentiates in gametes.