Twenty-five vital fluorescent dyes were tested for accumulation in haustoria of Erysiphe graminis (DC) Merat f. sp. hordei Em. Marchal when applied to coleoptile epidermal host cells of Hordeum vulgare L. Eight of the dyes gave fluorescence to structures judged to be haustorial mitochondria. The potentiometric cyanine dyes, DiOC4(3) and DiOC7(3) gave best differentiation between the putative mitochondria and other structures. As measured photometrically, fluorescence of the organelles diminished within a few minutes after DNP(1 × 10-4m), CCCP (1 × 10-6m), or KCN (1 × 10-3m) was applied to host tissue, indicating that fluorescence depended on the membrane potential of the organelles. Many of the tested dyes, including several cyanines, accumulated in haustorial cytoplasm (excluding mitochondria or vacuoles), but the uptake of most of these dyes was not affected by DNP or CCCP, indicating that the dyes moved passively into haustoria. However the uptake of the cyanines DiOC5(3) and DiSC2(3) was slightly reduced by CCCP suggesting that a component of uptake of these dyes depended on the potential of the plasmalemma or extrahaustorial membrane of the haustorium. The cyanine potentiometric dyes provide a way to monitor electrical potential across mitochondrial and other membranes which, in turn, may relate to functional competence of haustoria.