Aleuroplatus coronata (Quaintance 1900) and A. gelatinosus (Cockerell 1898) are native, sympatric whiteflies (Hemiptera: Aleyrodidae) that develop on live oaks, Quercus spp. (Fagaceae) in California. This study provides characters to separate them in all postembryonic stages. The phenology of these whiteflies was studied in Riverside County, California on one tree each of Quercus agrifolia Née and Q. engelmannii Greene. Their phenology was almost identical. Both species are univoltine. Adult emergence and oviposition are synchronized with the flush of new foliage in late March and April; however, egg hatch does not occur until late May or early June, and nymphal development occurs primarily after leaf maturation and hardening. Both species attain the final (fourth) nymphal instar by late summer and remain in this stage until adult emergence the following Spring. Both species were abundant on Q. agrifolia, but on the adjacent Q. engelmannii tree, only A. coronata was abundant. On Q. agrifolia, eggs of both species were laid on both the new flush and on the previous year's foliage, whereas on Q. engelmannii, oviposition was almost exclusively on the new flush. This corresponds to the longevity of the foliage; leaves of Q. agrifolia apparently survive two years, allowing development of a second generation of whiteflies, whereas leaves of Q. engelmannii last little over one year, and thus would be unsuitable for a second generation. The sympatric distribution, shared hosts and almost identical phenology give these whitefly species a very high degree of niche overlap. A. coronata usually dominates numerically over A. gelatinosus, but this varies among trees and among seasons.