Water toxicity monitoring based on the continuous cultivation of Photobacterium phorphoreum is presented. Normally, after 10 days of operation, a dark variant, which emits no light, appears and dominates the population, resulting in a rapid decrease in bioluminescence. Therefore, to overcome this problem, a fluidized-bed reactor is used in which alginate-immobilized cells are grown and leaking cells are continuously released into the effluent Experimental results revealed that the dominance of dark variants was suppressed inside the immobilized beads, thereby mitigating the rapid loss of bioluminescence. Plus, a high dilution rate (1.2 h$^$-1/) prevented the occurrence of other microbial contamination in the reactor The concentration and bioluminescence of the released cells were sufficient to measure the water toxicity for more than 4 weeks.