We examined a long life lactate sensor that employs two types of thermostable mutant lactate oxidase, both generated by random mutagenesis. One of the mutants, LOD15, exhibits an Asn to Asp point mutation at position 212, and the other, LODN1, exhibits a Glu to Gly point mutation at position 160 in the amino acid sequence. These LODs are shown to be more thermostable than wild-type LOD at 65°C, and their overall inactivation curve at 65°C is less steep. Although LOD15 lactate sensor output is higher than that of a wild-type LOD sensor at 24°C, over time during storage at 40°C, this output declines significantly. LODN1 sensor output, on the other hand, shows good linearity, and its output over the same storage time is about twice as high as that of a wild-type LOD sensor.