S U M M A R YPhysarum rigidum grew and sporulated in pure culture at pH 4-2 on a partially defined medium containing : mineral salts, glucose, yeast extract, haematin, casein hydrolysate. The inoculum equiv. 0.22 mg. protein/z5 ml. grew to equiv. 21 '5 mg. protein/25 ml. in about I 2 days. Physarumflavicomum varieties I and 2 also grew and sporulated on this medium. The yeast extract and haematin were essential for growth; the casein hydrolysate was not. Omission of glucose resulted in a 50% decrease in growth yield. Ethanol, galactose, glycerol, lactose, mannitol, mannose, potato starch, raffinose or sorbitol could replace glucose, but the growth yield was decreased. Fructose, inulin, sucrose and the pentoses and carboxylic acids tested did not support growth.

SUMMARYThree geographical collections (isolates) of myxomycetes originally identified as PhysarumJlavicomum were found to be heterothallic and the clones established from each isolate were divided into two mating types. Previous studies of six isolates of P.Javicomum showed that they could be divided into three groups. Mating occurred within groups but not between groups, and within each group there were two or more mating types. When the new isolates were crossed to the original isolates, they conformed to two of these three groups. This study established the existence of multiple alleles at the incompatibility locus in group 3. Mating-type alleles of the Fl generation of group 3 segregated in a I : I ratio at meiosis. Isolates belonging to group I were also separated from the other two groups on morphological and physiological bases. The morphological description fits that of P. rigidum more nearly than P. Jlavicomum and group I isolates were placed in the species P. rigidurn. Groups z and 3 were separated genetically and physiologically but morphologically the separation was not distinct. These groups were referred to as P.Jlavicomum varieties I and 2.