A cell biosensor specific for formaldehyde was developed using double-mutant cells of the methylotrophic yeast Hansenula polymorpha A3-11. The activities of some of the enzymes in the metabolic pathway of the wild-strain cells were deliberately suppressed by introducing respective genetic blocks to optimize the selectivity and acidification rate. Mutant yeast cells produced in this way were immobilized in Ca-alginate gel on the gate of a pH-sensitive field effect transistor. The local acidification of the extracellular medium due to specific conversion of formaldehyde was recorded. The steady-state response time of the biosensor was 2-3 min, i.e., about 10 times shorter than the response time for the alcohol-specific cell biosensors described earlier. The linear dynamic range of the sensor's response corresponds to formaldehyde concentrations of 2 to 200 mM. The operational stability of the sensor was not less than 4 h. The biosensor demonstrated high specificity to formaldehyde with no response to several organic acids, methanol, and other alcohols, except for low sensitivity to ethanol. The influence of sample buffer capacity and pH on the sensor response, as well as thermostability, was investigated.