The objective of this study was to determine the effects of different pretreatment agents such as chitosan (0.5% (w/v), pH 2.9-3.2) and lactic acid (0.5% (v/v), pH 2.5-2.7) on the chemical and sensory qualities of mussels stored at 4 degrees C. Mussels were dipped in 100 mL of 0.5% solution of lactic acid (v/v) and chitosan (w/v) at room temperature (22 degrees C) for 15 min. Mussels from the control group were dipped in 100 mL of sterile distiled water (2% NaCl) without chitosan and lactic acid. Treatment of mussels with lactic acid and chitosan at the beginning of the experiment (day 0) for 15 min reduced bacterial counts of total aerobic mesophilic bacteria (0.53-1.07 log) psychrotrophic bacteria (0.11-0.13 log), Lactobacillus spp. (0.46-1.30 log), Enterobacteriaceae (0.43-0.48 log) and coliform bacteria (0.52-0.66 log). Total volatile basic nitrogen (TVB-N), thiobarbituric acid (TBA), trimethylamine nitrogen (TMA-N) and histamine values of control group mussels were increased from (day 0) 13.1 mg N 100 g(-1), 1.37 mg MA kg(-1), 1.97 mg N 100 g(-1) and 4 ppm to 39.9 mg N 100 g(-1), 3.02 mg MA kg(-1), 4.86 mg N 100 g(-1) and 7.75 ppm at the end of the storage period (day 11), respectively (P<0.05). The results indicated that shelf-life of mussels stored at 4 C were limited to 4 days in the control group. However, mussels pretreated with lactic acid and chitosan were stored for 6-7 days and the shelf-life of mussels was extended for ca. 2-3 days, as compared with the control group (P<0.05).