A new species of mite, Acarophenax mahunkai Steinkraus & Cross, n. sp., was discovered parasitizing eggs of the lesser mealworm, Alphitobius diaperinus (Panzer), a serious cosmopolitan poultry pest. Acarophenax mahunkai is an obligate, apparently host-specific parasitoid that kills the egg upon which it feeds. Therefore, it may help control the lesser mealworm. The systematics and biology of A. mahunkai are presented here. In the laboratory, 51.0% of lesser mealworm egg masses were parasitized by one or more female mites. Male mites were not parasitic and had poor locomotory abilities. Significantly fewer lesser mealworm eggs hatched (23.9% egg hatch) in parasitized egg masses than in unparasitized egg masses (72.7%). Mite sex ratios were highly skewed, with means of 27.2 ± 3.9 female and 1.7 ± 0.2 male offspring produced per gravid mite. Physogastric female mites parasitizing lesser mealworm eggs gave birth to fertile adult females. The developmental time of the mites (3–5 d) was faster than that of lesser mealworm eggs (5–7 d).