Many animal species include populations composed of unequal numbers of both sexes, a phenomenon known as sex ratio distortion (SRD). SRD may be under maternal control, or may involve sex ratio distorting elements. The distorters are a class of selfish genetic elements enhancing their own spread by favouring the transmitting sex. In invertebrates, facultative SRD may result from extra-chromosomal effects, especially in female-biased populations infected with maternally inherited cytoplasmic micro-organisms (a-Proteobacteria: Wolbachia) and/or DNA fragments integrated in the host genome, probably as a transposon. The latter may feminize or kill embryonic males, enhance host fertility, or induce cytoplasmic incompatibility, male sterility, thelytokous parthenogenesis (= mother-to-daughter) or hybrid breakdown. In the case of pate rnally inherited distortion towards the male sex, nuclear transmission of the distorting factor is much more realistic for the mere reason that sperm cell cytoplasm, including extranuclear genes, is largely discarded during spermiogenesis. Nuclearly transmitted segregation distorters are often associated with B-chromosomes (= Bs or 'supernumeraries'). Further research is now being focused on the dosage effect of the Bs and their identification.