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The study was conducted to detect histidine decarboxylase-producing bacteria and determine histamine content for evaluation of potential histamine poisoning in 30 frozen specimens of longtail tuna harvested from the Oman Sea. Bacteriological isolates and the amount of histamine were obtained from 500 g of gill muscles. Trypticase soy agar was used for examination of mesophilic and psychrophilic counts. Enterobacteriaceae and histamine producing bacteria were enumerated in violet red bile dextrose agar and Niven's media, respectively. The results indicated that the average of total count and psychrophilic count were 4.81±0.26 and 4.66±0.25 Log10 CFU/g, respectively. Several bacteria were identified as histidine decarboxylase-producing bacteria. Among them, Clostridium perfringens had the highest contribution (24.4%) followed by Proteus spp. (23.0%), Klebsiella spp. (13.9%) and Enterobacter spp. (11.1%). Histamine contents in 20.0, 15.0 and 65.0% of samples were <20, 20-50, and >50 ppm, respectively. The high percentage of samples (65%) with histamine content higher than the standard (<50 ppm) suggests potential histamine poisoning due to longtail tuna fish consumption in humans. [ABSTRACT FROM AUTHOR]