A field study on the water quality parameters and prevalence of diseases was carried out in four locations in the Mekong Delta, Vietnam. A total number of 960 samples (816 with disease signs and 144 without disease sign) of farmed shrimp were collected over 8 sampling months (2 production cycles). Samples were collected every two weeks and when abnormal behavior from cultured shrimp was observed. Five major groups of gross clinical signs were recorded among diseased shrimp samples. Diseases that appeared through the sampling months were: (1) atrophy and pale-colored hepatopancreas (HP) with the empty or little food in the mid gut; (2) empty/little/discontinued food in the midgut and (3) slow growth, HP atrophy and gut with discontinued food. White feces disease was not found in April but appeared for the rest of sampling months with the highest prevalence in July (39,2%) and the lowest prevalence in September (4,9%). White spot disease appeared from October to December (at the end of when the monsoon season and low temperature).