We report the results from a mesocosm study investigating the interrelationship between microalgae and vibrios. The mesocosms were inoculated with plankton, plankton + sediment, or sediment. We followed the diatom bloom and increases in the abundance of Vibrio spp. and V parahaemolyticus in conjunction with several environmental variables in all mesocosms and at a reference site. The dominating diatom genera were also identified. Temperature, salinity, and pH were nearly invariant in the mesocosms and did not contribute to the results. The principal environmental variables that correlated to vibrio abundance were total bacterial plate counts, phosphorus and ammonia (positive relationship), and oxygen and silica (negative). Nitrate, total bacterial counts and chlorophyll a (chl a) did not correlate with vibrio growth. The highest diatom abundances were followed by increases in vibrios in all mesocosms. This was also observed in field sampling. Together, these results suggest that diatom blooms could support Vibrio spp. growth. V parahaemolyticus was initially favoured by sediment. The contribution of V parahaemolyticus to the total bacterial population was low, on average 0.5 %, but constituted a rather high proportion of the vibrio population in the mesocosm systems, i.e. on average 18 %. Some of the identified diatom genera, e.g. Chaetoceros and Skeletonema, were negatively correlated to vibrios, while Coscinodiscus was positively correlated. The results indicate that phytoplankton blooms, when recorded as high levels of chl a, should be used with caution as predictors for future vibrio epidemics, since the origin of the chl a might have a significant effect on vibrio abundance.