The multicapsid nucleopolyhedrovirus, SeMNPV, is naturally maintained in Spodoptera exigua populations by vertical transmission from females to eggs and by horizontal transmission from cadavers of caterpillars to other caterpillars. Laboratory and greenhouse experiments were conducted to quantify transmission rates and provide baseline data to assess the potential of this virus to maintain itself naturally in pest populations after application in biocontrol programs. As to vertical transmission, 18% of the first instar larvae originating from egg batches produced by infected moths contracted SeMNPV, while 34% of the egg batches gave rise to one or more infected first instar larvae. As only few large egg batches were virus-free, 70% of all uninfescted larvae hatched in egg batches that contained one or more infected larvae. Substantial horizontal transmisson was observed under greenhouse conditions. Presence of 0, 1 or 10 infected first instar larvae in a batch of 100 first instar larvae resulted in survival to the fifth instar of 41, 30 and 20%, respectively. However, the effect on crop injury was marginal. The observed high transmission rates favour the maintenance of SeMNPV in S. exigua populations in greenhouses.