A new allele at the W-locus ( W19H), found in a mutagenesis experiment in which females were irradiated, involves a presumed deletion. The deletion covers the Ph locus (which forms part of a gene complex with the W, Ph and Rw loci), and the locus of a recessive lethal 2 cM distal to W. It does not extend distally to the bl locus; nor does it involve the Rw locus, W19H/Rw compounds being viable and fertile. Thus, the length of the deletion is 2–7 cM. The non-involvement of Rw shows that, in the gene triplet Rw, W, Ph, Rw must lie proximal to W and Ph, whose relative position remains unknown. Heteozygotes for W19H are not anaemic, show only minimal white spotting and no pigment dilution; they thus resemble heterozygotes for the original W mutant allele and differ from W/Ph trans heterozygotes, which have extensive white spotting. In addition W19H heterozygotes may be small and runted, many are believed to die prenatally, and some in the nest. Their radiosensitivity is increased. Homozygotes die at the pre-implantation stage.