Dissertation ( MSc Biodiversity Conservation)Abundance and distribution of Fiddler crabs of genus Uca, were examined along the environmental gradient at Pangani estuary. Physio chemical parameters, abundance and distribution of Uca crabs were measured and observed respectively. Data were collected during a low spring tide through field observation and comparisons, were made along estuary river mouth (close to the sea), intermediate (at the middle between the sea and river) and upstream (close to the river). The results of physio chemical parameters showed that the organic matter and pH increased from the river mouth toward the upstream (p = 0.001, p = 0.0001 respectively) while salinity decreased from the river mouth toward the upstream (p = 0.0001), temperature did not show clear trend along the zonation with the lowest value at the intermediate and highest between two extremes (river mouth and the upstream). A total of 435 Uca crabs were counted and observed, out of which 91 (20.9‰) were from river mouth, 130 (29.8‰) at the intermediate and 214 (49.19‰) from the upstream. The results revealed that, the Uca crab abundance increased from river mouth towards the upstream as an organic matter and pH, however the intermediate site was the most diverse (H’= 1.76). Furthermore, results have shown that, there was unequal distributions of the species in the area, with only two species (U. vocans and U. lactea annulipes) were found on river mouth, four species in the intermediate site (U. tetragonon, U. urvillei, U. lactea annulipes and U. chlorophthalmus) and three species in the upstream (U. inversa, U. lactea annulipes and U. chlorophthalmus). In addition, the analyses showed that organic matter was the most important physio chemical parameter governing the abundance and distribution of Uca crabs (R² = 0.9211). These data suggest that there was a clear relationship in Uca crab abundance and distribution with the availability of organic matter, temperature, pH and salinity contents.