Two conditional mutants of Chlamydomonas reinhardtii, dif-1 and dif-2, affecting gametic differentiation under conditions of nitrogen (N)-starvation, have been isolated. These mutant cell remain "vegetative" at the restrictive temperature (35 degrees C) in -N medium, as defined by assays of cell body-agglutinin and cell wall lytic enzyme activities in the soluble fractions of cell homogenates. Moreover, the mutants fail to form mating structures at the restrictive temperature, but do so at the permissive temperature (25 degrees C). Temperature-shift experiments show that mutant cells which have differentiated into gametes at 25 degrees C dedifferentiate into "vegetative" cells under N-starvation conditions after transfer to 35 degrees C, but differentiate again into gametes at 25 degrees C. Genetic analyses indicate that the dif-1 and dif-2 genes are recessive and unlinked to each other or to the mating-type locus; the dif-1 phenotype cosegregates with a conditional flagellaless phenotype expressed in both +N and -N medium at the restrictive temperature.