Diapausing larvae of a laboratory and field strain of the tufted apple budmoth,Platynota idaeusalis(Walker), were reared on semisynthetic diet and maintained out of doors for observations between November 1980 and April 1981. Larvae maintained on semisynthetic diet continued to feed and molt during diapause, as long as temperatures remained consistently above 10°C. Quantitative estimates of feeding, larval weights, and head-capsule widths in diapausing larvae were also recorded. CO2output was measured in a laboratory strain of larvae reared on semisynthetic diet and excised apple leaves under long (16:18 [L:D]) and short (11:13 [L:D]) daylengths at constant 21°C. No increase in respiration was observed in any larvae immediately preceding larval–pupal ecdysis, regardless of food source or photoperiod. Diapausing larvae had lower respiratory rates than did nondiapausing larvae, particularly when comparisons were made based on larval weight classes, rather than chronological age.