The frequency of spontaneous cleft lip on Day 14 of gestation was examined in the A/J, A/HeJ, A/WySn, A.BY/Sn, A.SW/Sn, Bl0.A/SgSn, and B10.A(2R)/SgSn inbred strains of mice. A/WySn was found to have a higher frequency of cleft lip (20–30%) than A/J or A/HeJ (10%). Differences among strains in laterality of clefting and in rates of differential mortality of affected embryos were also found. Results of crosses between A/WySn and A/HeJ suggested that the difference in rate of clefting may be due to a genetic maternal effect. The results for A.BY/Sn, A.SW/Sn, Bl0.A/SgSn and B10.A(2R)/SgSn indicated that no association of cleft lip liability and the H-2 complex was detectable.