Samples of abnormal mud crabs, Scylla serrata (Forskål, 1755) (Decapoda: Portunidae), were collected from crab farms in Samutsongkhram Province, Thailand. These crabs had hard carapaces, red chelipeds and joints, pale hepatopancreas, gills, and soft muscles. They were almost immobile and finally died. The haemolymph revealed three stages of the syndrome, namely orange, orange-white, and milky-white in colors. The haemolymph, integument, hepatopancreas, gills, abdominal and claw muscle, stomach, and heart were dissected and histologically examined using transmission and scanning electron microscopy. Closer examinations found infection with rod-, curve rod-, or coccus-shape bacteria with thin and thick cell walls in all investigated organs and haemolymph. Isolation of the microorganisms from the infected tissues of red sternum syndrome crabs resulted in five types of bacteria. No microorganism growth was observed in normal crabs. Interestingly, the types of isolated bacteria can be classified according to the severity of the disease. Additionally, the degree of bacterial infection found was consistent with the stages of the disease. It was postulated that the bacteria entered the crabs via the gills, and then migrated through circulating haemocytes, before reaching the internal organs.