Twenty-five spp. of flagellate protozoa (in 2 orders, 8 families, 12 genera) live in the hind gut of the roach, Cryptocercus punctulatus. They all exhibit sexual behavior when their host molts. Between molts there is no sexual behavior, there is the haploid number of chromosomes, and division is mitotic. During molting their chromosome number is doubled. The change of mitosis to meiosis does not occur in all of them at the same time; some begin it in the early stages of molting, and others begin it 10-15 hrs. after the exoskeleton is shed. The sexual process is fertilization in Trichonympha, Leptospironympha, and Eucomonympha, but in each genus there are differences in the details of the process. In Trichonympha gameto-genesis occurs within cysts, each gametocyst producing 2 gametes. In Eucomonympha no cysts have been seen. In Saccinobaculus, Barbulanympha, etc., other processes occur. Fertilization is descr. in detail for each genus. Expts. indicate that the effect of molting on the protozoa is indirect.