In formulating artificial diets or rearing media for insect larvae, or in modifying natural larval media in attempts to regulate the insect population, the pH tolerance of the 1st-instar larvae must be determined to establish the optimum range for survival and also the conditions preventing production of larvae. A study was conducted at the Kerrville, Texas, Livestock Insects Investigations Laboratory to determine the pH tolerance of newly emerged (1st instar) horn fly larvae, Haematobia irritans (L.). Results are presnted here.