Summary: tclw5 is a t‐complex recessive lethal mutation of the t^w5‐haplotype. Since t^w5/t^w5 embryos die soon after implantation, the tclw5 gene is thought to play an important role in early embryogenesis. Previous histological studies have demonstrated that t^w5 homozygotes do not survive past the gastrulation stage due to extensive death of the embryonic ectoderm, whereas the extraembryonic tissues were less affected. In the present study, we demonstrate that t^w5/t^w5 embryos may be distinguished from wildtype littermates at embryonic (E) day 5.5. At this stage, the visceral endoderm of t^w5/t^w5 embryos appeared to be different, possessing smaller and fewer vacuoles compared to normal littermates. This led us to hypothesize that the visceral endoderm may be affected by tclw5. Confirmation was provided by the rescue of t^w5/t^w5 embryos following aggregation with tetraploid embryos. However, rescued embryos did not survive past E9.0 and displayed an underdeveloped posterior region. This would indicate that the actions of tclw5 extend beyond the midgestation stage. genesis 37:162–171, 2003. © 2003 Wiley‐Liss, Inc.