Atanycolus cappaerti Marsh & Strazanac (Hymenoptera: Braconidae) is a native North American parasitoid that has been found parasitizing late instars of the emerald ash borer, Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), a serious invasive pest of ash trees (Fraxinus species; Oleaceae). In this study, we determined the longevity, weekly oviposition rate, realized lifetime fecundity, diapause pattern, and development time of A. cappaerti for late instars of A. planipennis larvae at 25 ± 1.5 °C, 65 ± 10% RH, and a 16:8 h L:D photoperiod in the laboratory. Our study showed that adults of A. cappaerti had a median survival time of 9 to 11 wk, and females lived approximately 2 wk longer than males. The weekly progeny production by female parasitoids peaked at 3 wk after emergence, with a mean of 5.4 progeny per female. Throughout their life span, females produced a mean (± SE) of 28 (± 3.07) progeny. Atanycolus cappaerti larvae molted 5 times to reach the 6th instar in silk cocoons in a median of 8.3 d. Approximately 43% of the 6th instars continued development to adult wasps and the rest diapaused. The median development time from eggs to adults for non-diapaused parasitoids was 20 d. For diapausing A. cappaerti, however, the development stopped when larvae entered obligatory diapause in 6th instars. The median time for diapause-terminated larvae to adults was 28 d after being chilled for 2 to 4 mo at 2 to 4 °C. These findings provide critical information on the biology and life history of A. cappaerti that is important to the development of a potential mass-rearing protocol for augmentative biocontrol of A. planipennis.