Mendalian Inheritance in Cattle 1999 (MIC 99) is a comprehensive catalogue of those genes in cattle which have identifiable effects on the phenotype. The locus entries are presented in three categories: category 1 embreaces Coat Colour; Category 2 Visible Traits other than Coat Colour and Category 3 loci controlling blood and milk polymorphisms. Publications relating to the alleles at the locus are listed in chronological order for each locus entry, and an assessment was made of the reliability of the evidence presented for each allele. Category 4 relates to mapped loci and other genetic systems in cattle. As the genomic location of a locus is determined, the location is cross-referenced to the appropriate locus entry. As a result MIC 99 forms a link between the gene mapper and the cattle breeder. The breeder can consult MIC 99 to learn what a gene does, and, if its genomic location is known, where to find it. MIC 99 is the third catalogue in the COGNOSAG series, following a catalogue of genes with identifiable effect on the phenotypes of goats (COGNOSAG Workshop 1987) and Mendelian Inheritance in Sheep 1996 (MIS 96). Catalogues covering other domestic species are scheduled for the future.