The usage of lambda and kappa light chain variable region genes in peripheral fetal lamb B cells was assessed at intervals from 61 to 146 days of gestation (term = 150 days). Transcripts of 12 distinct Vlambda genes were identified, eight of which belonged to the previously described families I or II, while four segments defined a new Vlambda family, provisionally called family VI. A total of six different Vkappa gene segments were identified and grouped into four families, while two Jkappa gene segments were expressed. Southern blot analysis indicated that the sheep Igkappa locus probably contains approximately 10 Vkappa genes and suggests that the V gene cluster has been duplicated. Throughout fetal ontogeny, Vkappa transcripts contained more diverse CDR3 sequences than did Vlambda transcripts, where the V-J junctions were nearly always invariant and canonical. There was also a shift in usage of Vkappa genes and in the pattern of Vkappa-Jkappa gene recombination during fetal ontogeny.