The valve morphology of Mastogloia braunii Grunow, Mastogloia pumila Cleve, and Mastogloia vasta Hustedt were investigated using field material. Specimens for study were collected from Homa Lagoon, Izmir Bay, Turkey (4-29 degrees C; salinity 35%0-54%;)) as epipelic and epiphytic (Ulva lactuca L.). The valve morphological characteristics of the 3 species were measured by calibrated ocular micrometer using 20 specimens from each species per station. The valve of M. braunii was 35.15-80.20 mu m in length and 12.35-19.85 mu m in width, the valve of M. pumila was 13.70-31.85 mu m in length and 5.75-9.00 mu m in width, and the valve of M. vasta was 31.20-43.35 mu m in length and 11.90-18.10 mu m in width. The results of Pearson's correlation analysis revealed significant correlations among morphological features and environmental factors. All of the species display a positive correlation between valve length and valve width, and the number of transapical striae in 10 mu m are invariable with changes in environmental factors. Seasonal differences in the morphological features of Mastogloia species were determined by one-way ANOVA analyses. Valve length, valve width, and total number of partecta were the common characteristics showing seasonal variation.