Tests conducted at Mexico City during 1962 showed that continuous feeding of both sexually immature and mature adult Mexican fruit flies, Anastrepha ludens (Loew), on biotin at concentrations of 0.1%–2.0% in food resulted in loss of reproductive capacity in females. When biotin was removed from the diet, females recovered their ability to produce eggs. Treated male flies produced viable spermatozoa even though morphological aberrations of the gonads resulted. Mating frequency was generally low, and the higher doses caused mortality. Short-period feedings (12 days or less) on 2% biotin, followed by a return to standard diet, did not permanently suppress the reproductive capacity of either sex. Gonad development, mating behavior, and longevity were normal. Flies given a choice of food with or without biotin showed no preference.