ABSTRACT: Early Pliocene cheilostome bryozoans are preserved in a glacigenic diamictite at Cascade Cliffs, James Ross Island, northern Antarctic Peninsula. Several different marine bryozoan genera were incorporated into the diamictite during Pliocene ice advance(s). Bryozoan zooid-size Mean Annual Range of Temperature (zs-MART) analysis provides estimates of seasonality which suggest that mean annual marine temperatures for the James Ross Island region varied by at least 4.3°C and possibly by as much as 10.3°C during the Early Pliocene. This represents much greater seasonality for the northern Weddell Sea than seen at the present day (MART ca. 2°C). A cluster of zs-MART values between 6.6°C and 7.7°C from four colonies of the bryozoan Dakariella are considered to represent themost reliable range of seasonality estimates. Although we cannot determine absolute sea temperatures, increased seasonality signals an overall warmer climate for the Weddell Sea region at the time that the bryozoans were living.