A new mutation, Sleek, similar in appearance to mutations at the Ta, cr and dl loci, has been investigated. It is inherited as an autosomal dominant and maps very close to dl on chromosome 13. Allelism with dl seems probable since Sleek interacts with dl but not with cr. The unusual occurrence of dominant and recessive alleles at the same locus which produce a similar mutant phenotype suggests that the locus might either code for a multimeric protein or a regulatory product.