Recovery from CO₂ exposure (15 min at 14^oC) of most Dly¹/Dly¹ flies occurs between one and four hr, although many never recover, compared to less than 15 min for wild-type. Dly¹/+ flies recover between 15 min and one hr post-exposure. Percent of flies recovering decreases with increasing temperatures at time of exposure to CO₂ and with length of exposure. Exposure of Dly¹/Dly¹ flies to 28^oC during development rescues them but not their Dly¹-bearing progeny from delayed recovery. Dly¹ strains carry cytoplasmic factors which can transmit CO₂ sensitivity by injection. Cytoplasmically transmissible factor reduced by successive backcrossing, but Dly¹ strains remain CO₂ sensitive. Dly¹ strains show partial resistance to suprainfection by sigma virus.