ABSTRACTThe ultrastructural responses ofTaphrina deformansto 0.073μg ml-1propiconazole were studied as a function of incubation time with the inhibitor by transmission electron microscopy, and compared to those of the corresponding controls and an isolate that exhibited some tolerance to the inhibitor at 0.5μg ml-1. The principal responses relative to the control included: 1) thickened and layered cell walls with the outer layer being more electron dense and fragmented, 2) invaginated or convoluted plasma membrane, and 3) granules and membrane vesicles entrapped in the cell walls. These responses were apparent after 24 h incubation with the inhibitor, but not at 12 h, and became progressively more pronounced and frequent in cells with time up to 72 h. Partially tolerant cells exhibited essentially the same characteristics, but they were generally more pronounced than in the relatively sensitive cells. Growth of the cells in the presence of propiconazole either had no effect on cell wall glucosamine content (used as an indicator of chitin) or it resulted in an increase.