It has been shown by Cleveland (1930, 1948) and Cleveland et al. (1931, 1934) that the wood-feeding cockroach _Cryptocercus punctulatus_ depends upon certain intestinal protozoa for survival; these protozoa utilize as food the wood ingested by this cockroach. The wood is broken down into compounds the cockroach can utilize by the protozoa which elaborate a cellulase and possibly a cellobiase (Trager, 1932). Only molting nymphs of _Cryptocercus_ can pass the protozoa on to the newly hatched young, so that molting and hatching must happen concurrently each year or the young die. The sexual cycles in species of protozoa in the genera _Trichonympha_, _Saccinobaculus_, _Oxymonas_, _Monocercomonoides_, _Hexamita_, _Eucomonympha_, _Leptospironympha_, _Urinympha_, _Rhynchonympha_, _Macrospironympha_, and _Barbulanympha_ (fig. 3, B) are induced by hormones produced by _Cryptocercus_ only during its molting period (Cleveland, 1931, 1947, 1947a, 1949-1956a). Perhaps the prothoracic gland hormone of the host may be responsible for initiation of the flagellate sexual cycles (Cleveland and Nutting, 1955). The protozoan sexual cycles may be used as indicators of the onset of molting in _Cryptocercus_; thus different species of protozoa begin their sexual cycles from 35 days before to 2 days after molting of the cockroach (Cleveland and Nutting, 1954). Hollande (1952) and Grassé (1952) have reviewed the roles and the evolution of the flagellates in _Cryptocercus_ and in termites. The protozoa of cockroaches and termites are clues to the relationship between these two groups of insects. Kirby (1927) pointed out similarities between _Endamoeba blattae_ of _Periplaneta_ and the amoebae of the termite _Mirotermes_, suggesting that these protozoans were probably derived from an amoeba in an ancestor common to both blattid and termite. Kirby (1932, _in_ Kidder, 1937) found a species of _Nyctotherus_ in _Amitermes_ that resembles _Nyctotherus ovalis_ from domestic cockroaches. The belief that the termites and cockroaches had a common origin is also strengthened by the similarities between the hypermastigotes of both _Cryptocercus_ and termites (Cleveland et al., 1934). The cockroaches _Cryptocercus_ and _Panesthia_ both feed on wood, but the protozoa found in _Panesthia_ resemble more closely the species in domestic cockroaches than those in _Cryptocercus_. The Clevelandellidae (from _Panesthia_) are closely related to _Nyctotherus_ and have probably evolved from common ancestors. However, the separation of the Clevelandellidae from _Nyctotherus_ must have taken place at a later date than the divergence of their hosts, otherwise representatives of that family would probably also be found in _Periplaneta_ and _Blatta_ (Kidder, 1937).