Populations of Brachycaudus schwartzi (Borner) were found on peach and nectarine (Prunus persica Siebold & Zuccarini) trees in Parlier, CA, in summer 2000. To determine if this aphid could be a pest of P. persica, a nine-variety host range test was conducted using a no-choice arena and temperature-dependent development studies of the aphid at 15, 20, 22.5, 25, 27.5, 30, and 32.5degreesC. Nectarine variety Mayfire (r(m) = 0.286) and Queencrest peach (r(m) = 0.283) were found to be the best hosts followed by Mayglo nectarine (r(m) = 0.253). Mayfire was, therefore, selected as the host plant for the colony and for the temperature development studies. Detached shoot terminals of Mayfire were maintained on Hoagland's solution and placed in small cages in growth chambers. One adult aphid was introduced onto each flush and development of aphid offspring was monitored daily. The shortest developmental time from first instar to adult aphid was 6.9 d at 25degreesC and the longest was 19.9 d at 15degreesC. Adult longevity was 38.5 d at 15degreesC and the highest offspring per female was 46.4 nymphs per female at 20degreesC. The intrinsic rate of increase (r(m)) rose sharply from 15degreesC (0.109 nymphs/male/d) to a maximum at 25degreesC (r(m) = 0.286), and then fell sharply at 27.5degreesC with high mortality at 30degreesC and above. The lower threshold for development was 10.04degreesC. Overall, the data indicate that B. schwartzi does better at cooler temperatures and that early season peach or nectarine varieties may be susceptible to damage by this aphid.