The genomic organization, tissue distribution, and developmental expression of two paralogous GAPDH isoforms were characterized in the mud loach Misgurnus mizolepis (Cypriniformes). The mud loach gapdh isoform genes (mlgapdh-1 and mlgapdh-2) had different exon-intron organizations: 12 exons in mlgapdh-1 (spanning to 4.88 kb) and 11 in mlgapdh-2 (11.78 kb), including a non-translated exon 1 in each isoform. Southern blot hybridization suggested that the mud loach might possess the two copies of mlgapdh-1 and a single copy of mlgapdh-2. The mlgapdh-1 transcript levels are high in tissues requiring high energy flow, such as skeletal muscle and heart, whereas mlgapdh-2 is expressed abundantly in the brain. Both isoforms are differentially regulated during embryonic and larval development, during which their expression is upregulated with the progress of development. Lipopolysaccharide challenge preferentially induced mlgapdh-2 transcripts in the liver. Therefore, the two isoforms have diversified functionally; mlgapdh-1 is associated more closely with energy metabolism, while mlgapdh-2 is related more to stress/immune responses, in the mud loach.