The study of ostracod fauna of the coastal area near Phetchaburi allows the iden¬tification of 38 species. Taking into account the data of some environmental factors, it has been pos¬sible to recognize some ostracod groups which indicate relationships with depth and substrate. The application of Shannon Weaver (H') index demonstrates that the best conditions of species richness and equitability and, then, equitable environmental conditions are present in the central-southern sector of the investigated area. Conversely, stressing conditions occur in the northern sector in front of the Mae Khlong river mouth, which drains seaward pollutants and nutrients.
The sampling stations are included by us in categories corresponding to H' values. Except for those without ostracods (category 1 with H' not calculated), the sampling stations characterize other four categories. It is assumed by us that the environmental conditions improve from 1 to 5 category. When present, the ostracod species evidence in these categories some observations concerning their high or reduced capability to tolerate the variations of the environmental conditions. Species occurring in several categories are more tolerant than those present in a reduced number of categories. The for¬mer species can be labeled as opportunistic; the latter species can be considered "in equilibrium", or less opportunistic, at least. Thus, this work evidences that the ostracods can be good markers of the environmental variability, so becoming useful tools for the monitoring of a coastal area showing par¬ticular conditions linked to strong river influence and human activity
