Reciprocal crossing of the Mt Alford (A) strain of the cattle tick B. microplus with a susceptible (S) strain and phenotype analysis of F1, testcross and F2 progeny showed that high chlorpyrifos resistance in strain A was due to two genes that were complementary and jointly exhibited incomplete dominance. Diazinon resistance in the Gracemere (G) strain appeared to be similarly inherited. The 'average' degree of dominance ('average dominance', Dav) of high chlorpyrifos resistance over susceptibility, exhibited by F1 hybrids from A X S reciprocal crossings, was +0.54 on a -1 to +1 scale and was not significantly different from the parametric value of +0.5 (semi-dominance). The corresponding Dav values revealed by G X S crossings were +0.42 for diazinon resistance (significantly less than +0.5) and -0.031 for chlorpyrifos resistance (not significantly different from zero and therefore exhibiting zero dominance/recessivity). Resistance factors for chlorpyrifos in strains A and G for homozygotes were 74 and 35, respectively, and for F1 hybrids were 25-29 and 5-7, respectively. The resistance factors for diazinon in strain G for homozygotes and F1 hybrids were 174 and 37-41, respectively.