In the present study, we investigated the changes of calretinin (CR) expression in the central nervous system of SOD1G93A transgenic mice as an in vivo model of amyotrophic lateral sclerosis (ALS). In wild-type SOD1 (wtSOD1) transgenic mice, many CR-immunoreactive neurons were found in all cortical regions. In the cerebral cortex of SOD1G93A transgenic mice, the number and staining intensity of CR-positive neurons were decreased. In the hippocampal formation, layer-specific alterations in the staining intensity of CR-immunoreactive neurons were observed in the CA1-3 areas and dentate gyrus. In wtSOD1 transgenic mice, CR-immunoreactive neurons with long processes were found in the stratum oriens and stratum radiatum of CA1-3 areas, and heavily stained band-like molecular layer was prominent in the dentate gyrus. CR immunoreactivity was decreased in each layer of CA1-3 areas and dentate gyrus of SOD1G93A transgenic mice. The first demonstration of decreased immunoreactivity for CR in the cerebral cortex and hippocampus of SOD1G93A transgenic mice may provide insights into the pathogenesis of motor neuron degeneration in human ALS although further quantitative studies are needed.