Four microsatellite loci (MAF50, MAF18, OarFCB20 and MCM527) were studied in Rasa Aragonesa sheep in order to evaluate their use in paternity testing. Several population characteristics were estimated [allele frequencies. effective allele number (Ne), polymorphism informative content (PIC) and probability of excluding wrong paternities (Pe)]. In 32 randomly chosen individuals, four alleles were detected for MAF50, with 2.55 effective alleles, 0.58 PIC and 0.35 Pe. For MAF18, five alleles were identified, with 2.99 effective alleles, 0.51 PIC and 0.32 Pe. For oarFCB20, 10 alleles were observed, with 6.06 effective alleles, 0.82 PIC and 0.68 Pe. Finally, for MCM527, six alleles were found, with 3.75 effective alleles, 0.69 PIC and 0.50 Pe. When these loci were used together with serum transferrin locus, Pe rose to 97.20 per cent. Field trials confirmed the real usefulness of these techniques.