In this study, diurnal changes in bacterial production and DNA damage in bacterio-plankton (measured as cyclobutane pyrimidine dimers, CPDs) incubated in bags at different depths in tropical coastal waters were investigated. The DNA damage and inhibition of the bacterial production was highest at the surface and decreased with depth. Inhibition of both leucine (Leu) and thymidine (TdR) incorporation was detectable to a depth of 10 m, while DNA damage was measurable only until 5 m. In addition, the decrease of the inhibition of Leu and TdR incorporation with depth was less than the decrease in DNA damage. Variations in DNA damage in bacteria and in biodosimeters were mostly explained by UV radiation (UVR) dose received over time and depth. UVB; radiation (UVBR) contributed more than UVA radiation (UVAR) to the damage, The attenuation coefficient of DNA damage with depth was comparable to the attenuation coefficients of UVBR suggesting that DNA damage was induced mainly by UVBR. For incubations at the surface, there was a trend of increasing DNA damage at a constant rate during the entire period of daylight, while the inhibition of leucine incorporation showed a trend of a less rapid increase in the afternoon than in the morning. This might have been due to photorepair, which would also explain the lack of correlation between the inhibition of leucine incorporation and UV dose. It was concluded that CPD damage in DNA is not necessarily the predominant factor in the inhibition of bacterial production. It cannot explain the large impact of sunlight on bacterioplankton productivity in tropical waters.