**Abstract**

This chapter summarizes the developmental studies on environmental biosensors of enzymatic phosphate ion (Pi) biosensors for eutrophication and microbial biochemical oxygen demand (BOD) biosensors for organic pollution. In particular, an author focuses on the developmental studies that the author principally conducted, and describe the history and the insights into the future of these fields of environmental biosensors. In our developmental studies on the enzymatic Pi biosensors, we fabricated automatic instruments of a desktop-type and a submersible buoytype, which was fabricated for remote biosensing of dam water. These instruments employed a luminol-chemiluminescence flow injection analysis (CL-FIA) system and enabled to have practical performances in precise Pi determination, operational stability, and accurate bioavailable Pi measurements. In the microbial BOD biosensor development, the author considered to apply the FIA concept enabling highly repeatable measurements to absorptiometric BOD measurements. Both precise temperature control and accurate time control to incubate measurement mixture of budding yeast cell suspension containing redox color indicator and sample enabled to obtain the highly repeatable results that led to highly sensitive BOD measurements. Looking back on our developmental studies, what the author was thinking at the time and the results obtained are described. Finally, the author discusses the developmental trends of these biosensor fields and new insights into the future perspectives.

**Keywords:** biosensor instruments, eutrophication, organic pollution, phosphate ion, enzymatic biosensor, BOD, microbial biosensor, budding yeast, CL-FIA, accuracy, precision, automation, remote biosensing, submersible buoy
