X chromosomes derived from crosses of inbred P and M Drosophila melanogaster strains that had extreme effects on abdominal and/or sternopleural bristle number in males, were further analyzed to determine their effects in females and to map the loci at which the mutations occurred. Seven lines that had on average 3.9 fewer sternopleural bristles than wildtype in males had average homozygous sternopleural bristle effects of −2·2. The bristle effects were partially recessive, with an average degree of dominance of −0·60. Physical mapping of the sternopleural bristle effects of these lines placed them all at approximately 24·7 cM. These mutations are apparently allelic on the basis of a complementation test, and deficiency mapping indicates they occur within chromosomal bands 8A4; 8C6. In situ hybridization analysis of the sites of P element insertions of these lines suggests that mutations probably resulted from excision of P elements at 8C on the original inbred P strain chromosome. Two additional lines, NDC(19) and DP(146), had reduced numbers of sternopleural and abdominal bristles. NDC(19) males had 9·7 fewer abdominal and 8·6 fewer sternopleural bristles than wildtype. The corresponding homozygous abdominal and sternopleural bristle number effects were −5·8 and −3·8, respectively; with the abdominal bristle effect completely recessive and the sternopleural bristle effect nearly additive. DP(146) males had 6·2 fewer abdominal and 4·1 fewer sternopleural bristles than wildtype, with homozygous abdominal bristle effects of −4·3 and sternopleural bristle effects of −2·0. Abdominal bristle effects of this line were partially recessive whereas the sternopleural bristle effects were additive. Physical mapping showed effects on both bristle traits segregated jointly in these two lines, with the NDC(19) mutation closely linked to y and the DP(146) mutation 0·17 cM from it. Complementation tests and deficiency mapping also indicate the mutations in lines NDC(19) and DP(146) are at closely linked but separate loci within chromosomal bands 1B2; 1B4–6 and 1B4–6; 1B10 respectively, with some epistatic effects. In situ hybridization analysis of sites of P element insertion suggest that the NDC(19) mutation, which may be a scute allele, was probably caused by a P element insertion in the IB region; the DP(146) mutation is also associated with an insertion at IB.