In three multicentered field trials run during 1995 to 1997, involving a total of 12,928 lambs, the preventive effects on blowfly strike of a ready-to-use 5 per cent dicyclanil pour-on formulation and of a ready-to-use 6 per cent cyromazine pour-on formulation were compared with untreated controls for periods of up to 22 weeks. Their efficacy was assessed by calculating the consolidated cumulative strike rate for each treatment group and the consolidated percentage reduction of flystrike achieved by the two formulations. The consolidated cumulative strike rate for all three years together reached 29-2 per cent for the untreated control lambs at week 22, 2-8 per cent for the dicyclanil-treated lambs at week 22, and 6-9 per cent for the cyromazine-treated lambs at week 20. The consolidated cumulative percentage reduction of blowfly strike remained above 89 per cent for the dicyclanil-treated lambs. For the cyromazine-treated lambs the consolidated cumulative percentage reduction of blowfly strike reached 90 per cent during weeks 9 to 10, and 80 per cent during weeks 11 to 12.