In this paper we describe the behaviour of the symbiotic bacteria ofBlattella germanicaandPeriplaneta americanaduring embryo development using transmission and scanning electron microscopy. InBlattella germanica, the bacteria are transferred by endocytosis to the egg cell and are sited in its peripheral cytoplasm; thence they begin to internalize in the yolk, using cytoskeletic structures which are probably only synthesized at this particular phase of their migration. In the 6–7‐day‐old embryo, the bacteria are in close contact with the vitellophages scattered in the yolk, where some of them appear to be in a degenerative phase. InPeriplaneta americana, after the internalization of the bacteria, there is the formation of the mycetome which appears to be made up of a syncytial envelope, probably formed by the vitellophages, that contains a ball of symbionts of considerable number. During development, there is a progressive decline in the bacterial population, caused by complex lytic processes which take place inside the mycetome. In embryos aged more than 17 days, the mycetome tends to regress, while there are many bacteriocytes present. A hypothesis for interpreting this phenomenon is that the embryo may use the bacteria as nutrients. It cannot, however, be excluded that the formation of the mycetome may be the expression of a cellular immune process.