(1) Potential ethological isolating mechanisms were observed between breeds and strains of domestic fowl. (2) Brown and White Leghorn females exhibited homogamy, but females of a Broiler strain showed a preference for Breeding Line Brown Leghorn males, which bear a close phenotypic resemblance to the Red Jungle fowl (Gallus gallus spadiceus). (3) Observations indicated that females discriminated between males on physical characteristics rather than quantitative differences in male courtship. (4) In a ''choice-situation'', males which had been reared with own-breed females courted caged own-breed hens significantly more than others. Males reared with their own and other breeds showed only weak own-breed preferences, and in two cases, heterogamy was observed. (5) Males apparently discriminated between females on visual cues, particularly female plumage colour. (6) Assortative mating within a single line was also observed, females preferring some males to others. Quantitative differences in male courtship were not important in this respect. (7) In tests of short duration, males did not exert preferences for individual hens of a single line. (8) A sequential analysis of the courtship of the Brown Leghorn cock revealed that two displays were important in evoking female solicitation, namely the waltz and the rear approach. Three other displays were interpreted as increasing sexual arousal in the female. (9) The evolutionary significance of some of the results is briefly discussed.