The stage at which homozygotes die was determined for 28 mutations (general symbol c*) at the albino (c) locus, of which 26 had earlier been found to be probably prenatally lethal. Within each of the mutant stocks, the uterine contents of c*/cch females, made pregnant either by c*/cch males ("Ex" series) or by cch/cch males ("Co" series), were examined between 13 and 17 days postconception. Altogether, 743 females were dissected and 7197 corpora lutea (representing ovulations) counted. In selected stocks, an additional 40 and 13 females were dissected on days seven or nine, respectively.—In each of the 26 presumed prenatally lethal mutants, there was a deficiency of living fetuses in the Ex, as compared with the Co, group. Overall, this deficiency was 23.6% (expectation, 25% c*/c*). All meaningful excesses were in numbers either of moles (death shortly before, during, or just after implantation), or of early preimplantation losses. Homozygotes in none of the mutant stocks die between days nine and 19 postconception. Of 24 c-locus mutants known to be deficiencies since they lack the closely linked Mod-2, 13 clearly kill before implantation, ten around implantation, and one neonatally. The c and Mod-2 loci and the region between them are not needed for intrauterine survival.—There are indications that the distinction between early-preimplantation death and implantation death may, in a general way, be related to length of the deficiency.