This study examined the event ofprotein phosphorylation in bovine oocytes in response to sperm penetration and parthenogeneticactivation. In vitro matured oocytes were labelled with [32P]orthophosphate at 3 h intervalsfrom 3 h to 18 h or from 0 h to 12 h following in vitro fertilisation and parthenogeneticactivation, respectively. The level of protein dephosphorylation, at approximately 43 kDa, was similarin fertilised and parthenogenetically activated bovine oocytes. However, the level of proteinphosphorylation at 40 kDa, 23 kDa and 18 kDa was different between these two samples. There were nosuch changes of protein phosphorylation and dephosphorylation in the control oocytes. Further, bytwo-dimensional gel electrophoresis there is a difference in the level of protein phosphorylation at18 kDa between the fertilised and activated oocytes. These results suggest that this proteinphosphorylation may be related to the formation of the male pronucleus in bovine oocytes.