Techniques were developed to assess germplasm from wild and commercial brassica accessions for host plant resistancet o Aleyrodesp roletella. Several wild accessionsp ossessedr esistancei n both laboratory and field experiments. Most varietal differences in preference for Brassfca oleracea cultivars were due to the host plant morphology. Resistance conferred by glossy leaves was quantitatively shown for the first time. Laboratory studiess howedt hat resistancein B. incana andB . villosa was based on an antixenotic mechanism conferred by dense downy hairs which significantly reduced oviposition. Resistance in B. spinescens and B. fruticulosa was based on an antibiotic mechanism which significantly reduced the life span of adult whiteflies and increased the mortality of the larval stages. The feeding of A. proletella was assessedu sing electrical penetration graph techniques. Feeding duration was shorter on B. fruticulosa than the susceptible control. In addition, A. proletella styletsd id not penetratem esophylcl ells,t he importanceo f this in relationt o virus transmission is discussed. Stylet activity of larval whiteflies was greater on both B. spinescens and B. fruticulosa than a susceptible control indicating problems in finding or remaining within the phloem. Some aspects of the relations between larval whiteflies and their hosts are discussed