Factors influencing the development of the alatoid form of the brown citrus aphid, Toxoptera citricida (Kirkaldy), were investigated in the laboratory using Carrizo citrange as a host plant. No evidence of maternal influences on wing development was observed and nymphs were not irreversibly committed to wing development until the late second or early third instar. Both alatae and apterae were significantly larger when they developed on young, flushing terminals citrus than on older, hardening terminals. Similar numbers of late second/early third instars from high-density colonies developed wings when transferred to young and old citrus terminals, but mortality was significantly higher among aphids on the older shoots. When all adults were harvested daily from high-density colonies the percentage of aphids maturing into alatae reached a peak after about 4 d and then declined with aphid density, despite the advancing age of the plant tissues. The density of aphids in a colony was positively correlated with the proportion maturing into alatae.