The occurrence, development and histopathology of monodon baculovirus (MBV) in larvae, post-larvae (PL) and broodstock of Penaeus monodon was studied over a period of 1 year in southern Thailand. In histological samples of obtained from the Andaman Sea, the incidence of individuals with MBV occlusion bodies (OB's) was similar to 5.7%. Absence of MBV virions in electron micrographs of oocytes of MBV-infected females, mature eggs and nauplii indicated that the virus may not be transmitted transovarially. Together, these facts suggest that elimination of MBV from hatcheries by screening of quarantined wild broodstock would be feasible. It was possible to follow groups of PL from the nursery stage through to harvest of market-size shrimp. Average survival data for such shrimp followed in a large number of ponds over a period of 1 year indicated that MBV was well tolerated by Penaeus monodon if other rearing conditions were optimal. However, in the event of environmental or other stress, the incidence of animals showing large numbers of MBV particles increased dramatically.