A cDNA probe for the β subunit of bovine FSH (FSH-β) detects multiple restriction fragment length polymorphisms (RFLPs) in sheep genomic DNA consistent with an insertion/deletion polymorphism around the FSH-β locus. The presence of the insertion/deletion was confirmed by screening over 100 individuals with two restriction enzymes detecting RFLPs. All individuals showed the same patterns of fragments with both enzymes. A partial restriction map of the FSH-β gene in sheep suggests that the insertion/deletion is approximately 2 kb in size and located downstream from the third exon. Individual DNA samples were analysed from two flocks where the Booroola F gene is known to be segregating. Individuals that were heterozygous for the F gene were shown to be homozygous for one or other of the two alleles. Genetic recombination between the FSH-β locus and the F gene was observed in four pedigrees and there was no evidence that the insertion/deletion is closely linked genetically to the Booroola F gene. A major gene transcript of 2·2–2·3 kb was detected on Northern blots of sheep RNA. Neither the insertion/deletion polymorphism nor the presence of the F gene appeared to influence the size of the FSH-β gene transcript.