Paternal-Sex-Ratio (PSR) is a B chromosome that causes all-male offspring in the parasitoid wasp Nasonia vitripennis. It is only transmitted via sperm of carrier males and destroys the other paternal chromosomes during the first mitotic division of the fertilized egg. Because of haplodiploidy, the effect of PSR is to convert diploid (female) eggs into haploid eggs that develop into PSR-bearing males. The PSR chromosome was previously found to contain several families of repetitive DNA, which appear to be present in local blocks. PSR chromosomes with irradiation-induced deletions have decreased rates of transmission and increased variation in transmission. This study investigates whether these differences in transmission of deletion chromosomes are due to mitotic instability. Two deleton chromosomes (E306 and F316) and the wild-type PSR chromosome were examined. A cytogenetic assay of testes revealed that wild-type PSR males contained the chromosome in 98%-100% of their spermatocytes. Similar counts from carriers of two delection chromosomes were lower and varied between individuals from 50%-100%. One F316 male did not contain the chromosome in any of its spermatocytes although the chromosome was present in somatic tissues based on hybridization to PSR-specific repetitive DNA. A molecular analysis of males found the wild-type PSR chromosome to be present in all somatic tissues. Tissue specific differences in the presence of PSR were found in several males from the two deletion lines. The results show that deletions can result in mosaicism due to increased mitotic instability of PSR. Such individuals sometimes partially or completely fail to transmit the chromosome. Patterns of mosaicism of B chromosomes in other organisms are discussed.