|Abstract||The effectiveness of polymer matrix tags containing (w/w) 8.5% cypermethrin, 7.5% flucythrinate, 13.7% tetrachlorvinphos or 20.0% diazinon in protecting sheep against head strike by the sheep blowfly (Lucilia cuprina Wiedemann) was investigated in larval implant, fly cage and field studies. Tags impregnated with cypermethrin reduced the total number of egg masses deposited on the heads of sheep in fly cage studies over a 6 week period by 7 3.3% compared with no treatment. Tags impregnated with flucythrinate reduced the number of egg masses by 25.3% over 21 weeks but there were no significant differences (P < 0.05) between treated and untreated sheep at individual exposures. Egg masses were found on the majority of tagged sheep and no protection was provided against implants with first instar L. cuprina larvae by either cypermethrin or flucythrinate tags. Tags impregnated with diazinon gave longer protection than treatment with a liquid formulation containing 400 ppm diazinon in larval implant, fly cage and field studies. Over a 12 week period in field studies, 6.6% of rams treated with diazinon tags became struck compared with 30% treated by diazinon jetting, 35.7% treated with plastic tags not impregnated with insecticide and 24.2% of untreated rams. When the rams were exposed to high populations of L. cuprina in an exposure house from 13 to 18 weeks after treatment, 3.3% of rams treated with diazinon tags, 57.1% treated by diazinon jetting, 43.8% treated with plastic tags and 23.5% of untreated rams became struck. Most strikes in the diazinon tagged sheep occurred at sites which were not contacted by the tags. Tags impregnated with tetrachlorvinphos reduced the number of strikes in comparison with no treatment in larval implant and fly cage studies but the results were inconsistent and not as good as those from diazinon tags. It is concluded that well designed controlled release devices that reliably contact the wool on the heads of sheep at sites of flystrike risk and which are able to withstand damage from rams fighting may be able to give prolonged protection against head strike.