ABSTRACT: Net primary production and respiration were estimated in a hypersaline cyanobacterial mat colonizing a gypsum crust in the Eilat salterns, Israel. Two different approaches were used: in situ microprofiling with Clark-type O₂ sensors and application of optode sensor spots in incubation chambers. The net O₂ release rates of the mat phototrophs was high, with a maximum of 3.4 nmol O₂ cm^–2 min^–1 measured by microprofiling and 4.4 nmol O₂ cm^–2 min^–1 determined in the incubation chambers. The upper 2 layers of the mat as well as the overlying water quickly became O₂ saturated during the day. The respiration of the whole gypsum crust was also very intensive and corresponded to the O₂ produced by photosynthesis on a diurnal basis, which prevented most of the evolved O₂ from reaching the water. The results presented show that optode sensor spots are useful tools providing additional information about export and photosynthetic production rates of O₂ in hypersaline microbial mats.