SUMMARYGrowth in defined media of 32 populations ofGonium pectoralewas studied to learn more about the sexual isolation reported for this species. The 23 populations containing both mating types(+&‐)were also studied for the ability to form zygotes in defined media. A preliminary study showed that some populations grew and reproduced sexually in a defined mineral medium, whereas others appeared to require exogenous organic materials for growth and/or zygote formation. The diverse reactions exhibited by the populations indicate physiological races which may explain, in part, the occurrence of sexual isolatio