This paper reports detailed information about fluctuations in catches of grey mullet (Mugil cephalus) in relation to oceanographic conditions in the waters west of Taiwan. The fishing ground for grey mullet was primarily located in the coastal waters from Tungyin to Wuchi. Gill nets and purse seines are the most-effective fishing gear used for catching grey mullet. Reasons for decreases in the catches of grey mullet were bad sea conditions and increased catches by mid-water trawl nets used by mainland Chinese fishermen.The ages of grey mullet captured this year were mostly 3 and 4 years old. In addition, females were usually larger than males. The resident and migratory grey mullet in the western waters of Taiwan cannot be separated morphologically. Allozyme analysis was applied to estimate genetic variation between the two localities of migratory grey mullet and to test whether they belong to the same population. The gonadosomatic index and the mean of oocyte diameter of the migratory types of grey mullet in the fishing period were greater than those which were not caught in the fishing period.