Seasonal and spatial variations of the oceanographic conditions in Hirosima Bay were characterized using the results of the field observations from 1989 to 1993. In Hiroshima Bay, the north-south enviromental gradients from the head to the entrance of the bay were observed. From the gradients, the bay is classified into two major regions, the northern and southern regions, by Nasami Strait where is located between Itsuku Is. and Nishi-Noumi Is. In the northern region, the seasonal variations in the oceanographic factors were remarkable and the density stratification developed in summer. In this region, the north-south enviromental gradients from the estuary of the Ohta River to Nasami-Strait were especially remarkable. This result indicated that this region is very stagnant and highly eutrophic. While, in the southern region, there was the west-east density gradient of surface sea water in the horizontal distribution. On the other hand, the horizontal distribution of chlorophyll a was roughly homogeneous, which shows relatively small influence of the nutrient loads from both the west and the north. Therefore, it is suggested that the southern region would be strongly influenced by the outside waters, in particular, the water of Aki-Nada adjacent to Hiroshima Bay.