The potential of two bioinsecticidal formulations containing Brevibacillus laterosporus spores and azadirachtin, respectively, was assayed in laboratory and in comparative field treatments for the management of immature house flies on dairy farms. As already known for B. laterosporus, preliminary laboratory experiments with azadirachtin evidenced a concentration-dependent effect. Azadirachtin median lethal concentration (LC50) value determined for second instar larvae was 24.5 µg/g diet. Applications in dairy farms were performed at dosages and concentrations predetermined in laboratory experiments, to employ the two formulations at an equal insecticidal potential. Repeated applications on the cow pen caused a significant fly development depression in areas treated with azadirachtin (63%) and B. laterosporus (46%), compared with the control. Formulations were applied at a dosage of 3 liters/m2, and concentrations of 2 × 108 B. laterosporus spores/ml and 25 µg azadirachtin/ml, respectively.