Measurements of fish and blue-green algae distributions were performed simultaneously in the shallowSulej๓w reservoir (Poland) to check for their coexistence in space and time. An EK 60 Simrad echosounderwith a horizontally beaming 200 kHz transducer was used to record the fish population, while a phyco-cyanin fluorescence on-line detection method was used to estimate the cyanobacterial biomass. Maps ofthe fish population and cyanobacteria cell distributions sampled every 20 s were produced using krigingtechniques. The subsequent analysis of the distribution maps showed that concentrations of fish andalgae sometimes do overlap in space and time, thus increasing the risk of fish contamination and humanmorbidity. More studies are required to further explore the relationship between fish and cyanobacteria.