Goat αs 1 -casein, coded by the CSN1S1 gene, is a highly polymorphic protein. The E allele is characterized by the insertion of a long interspersed nuclear element (LINE) in the 19th exon of the gene. This insertion is probably responsible for the reduced levels of αs 1 -casein found in milk from animals expressing the E allele. The present study aimed to develop a robust method for the detection of the CSN1S1 E allele in goat genomic DNA, using a one-step allele-specific polymerase chain reaction (AS-PCR). Three primers were designed, based on published DNA sequences (GenBank: AJ504710 and X72221). The primers were used simultaneously in the amplification reaction. Genomic DNA samples from animals with known CSN1S1 AA, EE and AE genotypes were used as positive controls. The DNA fragments were analysed by agarose gel electrophoresis, and were found to be of the expected sizes. The fragment characterizing the A⁎ allele (A⁎ refers to the αs 1 -casein alleles bearing an intact 19th exon) was 583 bp long, and the fragment characterizing the E allele was 437 bp long, spanning part of the 18th intron, part of the 19th exon and 146 bp of the LINE inserted sequence. This method allows clear identification of the three genotypes of CSN1S1 (A⁎A⁎, A⁎E and EE) using a one-step PCR. Using this method, the frequency of the CSN1S1 E allele in a population of 300 Sarda goats was estimated to be 0.037.