Diatoms are the earliest autotrophic colonizers and are responsible for the major input of energy in the form of reduced carbon to the surfce. However, information regarding the population structure of diatoms in theearly phases of fouling is very limited. In the present study, enumeration of fouling diatom population was carried out by exposing different types of substrata for four consecutive days at the sub-surface level. The study was carried out at Dona Paula bay located at the mouth of the Zuari estuary along the west cost of India. Quantification of the attached organisms from different substrata and from the surrounding environment was carried out microscopically and by the chlorophyll a estimation. navicula sp. was the most abundant form encountered. The paper deals with the qualitative and quantitative aspects of diatom colonization, their community structure, and correlation between the prevailing diatom population in the environment and that in the fouling community