Embryo-lethal mutants of Arabidopsis thaliana were isolated by treating mature seeds with an aqueous solution of ethyl methanesulfonate (EMS), screening the resulting M-1 plants for siliques containing 25% aborted seeds following self-pollination, and verifying the presence of induced mutations in subsequent generations. Thirty-two recessive lethals with a Mendelian pattern of inheritance were examined in detail. Developmental arrest of mutant embryos ranged from the zygotic stage of embryogenesis in mutant 53D-4A to the linear and curled cotyledon stages of development in mutants 112A-2A and 130B-A-2. These lethal phases did not change significantly when plants were grown at 18 °C rather than at 24 °C. Differences between mutant lines were found in the color of arrested embryos and aborted seeds, the percentage and distribution of aborted seeds in heterozygous siliques, the size of arrested embryos, and the extent of abnormal development. Unusual mutant phenotypes included the presence of unusually large suspensors, distorted and fused cotyledons, reduced hypocotyls, and arrested embryos without distinct cotyledons or hypocotyl tissue. The isolation of eight new mutants with a non-random distribution of aborted seeds in heterozygous siliques provides further evidence that many of the genes that control early stages of embryogenesis in plants are also expressed prior to fertilization.