Single Comb White Leghorn pullets were fed cyromazine (Larvadex) continuously at levels of 0, 25, 250, and 1,000 mg/kg diet (ppm) from hatch to 20 wk of age. Fresh manure was bioassayed for toxicity to housefly, Musca domestica, larvae beginning at the 6th wk after removal of cyromazine from the feed, and at weekly intervals thereafter. At 6 wk after removal of the feed additive there was 51.6% fly mortality at 25 ppm, 75.7% at 250 ppm, and 86.5% at 1,000 ppm relative to the 0-ppm control. Fly mortality decreased to less than 10.7% mortality at 13 and 15 wk postremoval for hens grown on 25 ppm and 250 ppm cyromazine, respectively. Hens grown on 1,000 ppm cyromazine produced manure that was still exhibiting more than 50% fly mortality 20 wk after removal of the feed additive. These data demonstrate retention of cyromazine in laying hens for up to 20 wk after feeding the chemical to the birds at 5 to 200 times greater than the maximum recommended rate for the initial 20 wk of life.