Aims: To assess the effects of soil type and time of the year on survival of the biocontrol inoculant Pseudomonas fluorescens CHA0 under aerobic conditions in lysimeter effluent water. Methods and Results: Effluent water was collected at different times from large outdoor lysimeters (2.5 m deep), which contained a well-drained or a poorly-drained cambisol, and inoculated with CHA0. The inoculant was monitored for 175 d by colony counts, total immunofluorescence cell counts and Kogure's viable cell counts. Cell numbers obtained with the three methods were similar. The inoculant declined exponentially in time and its population level varied considerably depending on the time of the year at which effluent water had been collected and soil type in the lysimeter. Positive correlations were found between the number of resident culturable aerobic bacteria and subsequent survival of the inoculant. Conclusions: The fluctuations of inoculant survival patterns correlated with differences in biological properties of lysimeter water that were related to soil type and time of the year. Significance and Impact of the Study: Results suggest that predictability of the survival of bacterial soil inoculants transported to groundwater level by heavy rainfall may be improved by taking into account key biological properties of the water.