The in vitro antihepatitis B virus (HBV) activities of eleven novel imidazo[1,2-c]pyrimidin-5(6H) one dideoxynucleoside analogues in which the sugar ring is 1,3-dioxolane or 1,3-oxathiolane were compared in the chronically HBV-producing human cell line 2.2.15. Seven nucleoside analogues 4, 9, 10, 15, 16, 18, and 24, of which 16 possesses the trans relative stereochemistry, displayed good potency and selectivity towards HBV. The order of decreasing potency at the 90% extracellular DNA inhibition level was 15>16>24=9>10>18. None of the tested imidazo[ 1,2-c]pyrimidines inhibited the replication of HIV-1 in MT-4 cells.