The strikingly beautiful Asterorotalia trispinosa, which is probably endemic to the Bay of Bengal, has long evoked interest among foraminiferologists, especially with respect to its ecology and limited distribution. Although this species has been observed in samples from sandy beaches, marginal lakes, and the inner shelf off the east coast of India, its micro-habitats are poorly known. This study provides important insights into the ecology of A. trispinosa as observed and recorded from the Muthupet Lagoon, a unique, shallow, marginal-marine ecosystem on the southeast coast of India, where salinities are only slightly lower than normal marine (~29-33%o). In this very shallow, mud-dominated (~95%) environment, where winds and microtidal currents maintain fine sediments continuously in suspension, an A. trispinosa population overwhelmingly dominates the very sparse foraminiferal assemblage (1-2 live specimens and 10-20 tests/g sediment). Typical estuarine taxa are rare to absent in Muthupet Lagoon.