Water samples were taken at 6 stations from the mouth of Keum River to Kogunsan Archipelago of West Sea during December, 1991 to August, 1992, to determine the distribution of heterotrophic bacteria, their biovolumes and heterotrophic activities. Heterotrophic marine bacteria ranged from $1.0;{ imes};10^3to;5;{ imes};10^5c.f.u.$ /ml. As for morphological distribution measured by epifluorescence microscopy, rod-shaped bacteria were between 45% and 72% of all cells during investigation period. Average biovolume of sampled bacteria ranged from $(7.69;{pm};0.18);{ imes}10^{-2}to;(8.18;{pm};0.38);{ imes};10^{-2};{mu}m^3$ for coccoid bacteria, and from $(6.09;{pm};0.29);{ imes}10^{-2}to;(7.72;{pm};0.41);{ imes};10^{-2};{mu}m^3$ for rod-shaped ones. The activities of extracellular enzymes ranged from 0.01 to 2.6 ${mu}M$ /l /hr for glucosidase, from 0.01 to 2.6 ${mu}M$ /l /hr for amylase, from 0.01 to 8.86 ${mu}M$ /I /hr phosphatase and from 0.01 to 0.94 ${mu}M$ /l /hr for chitinase. Extracellular enzyme activities were higher in summer season than in other sampling periods, and phosphatase showed the highest activity among measured extracellular enzymes. Bacterial distribution and their extracellular enzyme activities were associated with water temperature and organic nutrients, but bacterial cell volumes showed no direct relationship with extracellular enzyme activities.