House flies Musca domestica L. were presented four different intensity levels of competing fluorescent light. Their response to a UV light trap was measured. Flies were also presented four different sources (qualities) of competing light and their response to a UV light trap was also measured. All treatments were compared to concurrent dark controls with no competing light sources. Results indicate that as intensity of competing light sources increases, the catch efficacy of insect light traps decreases. Implications for fly management in urban areas are discussed.