Aging of bovine dung for 3–5 years under refrigeration resulted in smaller sizes of horn fly, Haematobia irritans (L.), pupae. Growth of horn fly larvae in mixtures of aged and fresh dung resulted in production of larvae and pupae stunted in proportion to the relative amount of aged dung. Reduction of size of pupae produced in aged dung was because of nutrient limitation, which was partially reversible by supplementation with a bacteriological medium or yeast extract that increased the numbers of bacteria for consumption by horn fly larvae. Similar stunting of growth and development of horn fly larvae was produced by crowding larvae in fresh bovine dung. Stunting was dependent on the amount of larval crowding, with greater larval densities resulting in smaller individuals but greater aggregate pupal weight, suggesting that stunting at great larval density resulted from phagoinhibition rather than depletion of nutrients in the fresh dung. Successful eclosion of adult horn flies was dependent (0.9578 correlation) on pupal size between 1.0 and 2.7 mg. The smaller size limit for pupae that were successful in eclosion of adult flies was about 1.0 mg, compared to an average weight for horn fly pupae of 4.2 mg. Implications for modeling and control of horn flies are discussed.