Abstract Cytoplasmically (maternally) inherited bacteria that cause reproductive incompatibility between strains are widespread among insects. In the parasitoid wasp Nasonia, incompatibility results in improper condensation and fragmentation of the paternal chromosomes in fertilized eggs. Some form of genome imprinting may be involved. Because of haplodiploidy, incompatibility results in conversion of (diploid) female eggs into (haploid) males. Experiments show that bacterial density is correlated with compatibility differences between male and female Nasonia. Males from strains with high bacterial numbers are incompatible with females from strains with lower numbers. Temporal changes in compatibility of females after tetracycline treatment are generally correlated with decreases in bacterial levels in eggs. However, complete loss of bacteria in mature eggs precedes conversion of eggs to the "asymbiont" compatibility type by 3-4 days. This result is consistent with a critical "imprinting" period during egg maturation, when cytoplasmic bacteria determine compatibility. Consequent inheritance of reduced bacterial numbers in F1 progeny has different effects on compatibility type of subsequent male vs. female progeny. In some cases, partial incompatibility occurs which results in reduced offspring numbers, apparently due to incomplete paternal chromosome elimination resulting in aneuploidy.