In attempts to determine the causal agents of blackcurrant reversion (BCRD) and gooseberry vein banding (GVBD) diseases of Ribes species, details of the ultrastructure of different kinds of tissue from plants affected with these diseases were studied. In 3 of 12 black currant plants affected with BCRD, leaves and flowers of plants showing symptoms typical of the severe form of the disease, contained rhabdovirus-like particles c. 65-80 nm x 215-485 nm. They were seen most often in the nucleus of cells as single particles but were also found in clusters or rafts. In leaves, these virus-like particles (VLPs) were present only in cells associated with the xylem parenchyma where they occurred as membrane-bound clusters within the nucleus. In flowers, they were also found in phloem parenchyma cells in the peripheral cytoplasm and very occasionally in the cytoplasm of epidermal cells. All non-nuclear VLPs were membrane-bound, either singly or in groups and the membrane seemed to be part of the endoplasmic reticulum. The proportion of vascular cells containing these VLPs was very low (<1%). In a few cells, smaller bacilliform particles, c. 40-50 nm x 200-250 nm, were found in the nucleus together with the larger particles. Double-membrane bodies, detected in fig leaves affected with fig mosaic (the agent of which is also mite-transmitted), were not detected in any BCRD-affected plants. In leaf tissue of 1 of 3 gooseberry and 1 of 2 blackcurrant plants affected with GVBD, 2 VLPs were found. Rhabdovirus-like particles, similar to those in BCRD-affected material, were present in the nuclei, perinuclear space and cytoplasm of xylem parenchyma cells. They were c. 60-72 nm x 155-230 nm but there was no evidence of the smaller rhabdovirus-like particles detected in a few cells of BCRD-affected tissues. The second kind of VLP was found in non-crystalline masses, with a mean centre-centre spacing of c. 10 nm, in the cytoplasm of phloem cells. These particles, together with other ultrastructural changes, were typical of those reported for aphid-transmitted closteroviruses. No badnavirus-like particles, reported previously from GVBD-affected plants, were observed in any of the plants studied. The significance of these findings in relation to these 2 important diseases of commercial Ribes species is discussed.