The Pembroke Power station has been in operation since September 2012. Since then, it has come under scrutiny regarding the use of the Milford Haven estuary water in its cooling system. The outflow of water from the cooling system is deposited back into the estuary at roughly 8oC higher than normal. Previous research into intertidal species has seen that high temperatures often affect the metabolism of these species. This study looks into the effects of thermal pollution on the gastropod, Gibbula umbilicalis, which is also known as the Purple Topshell (Fish et al, 2011; Pizzolla, 2008b). The effects are studied by completing monthly surveys of four sites situated at different distances from the outflow pipe; the outflow pipe itself, Pembroke Ferry, Jetty Beach and Broad Haven North. The surveys consist of quadrat sampling and visual counts of the Topshell in question. Thirty Gibbula umbilicalis¿ were removed from each site every month for dissection, where the width of the gonad, sex and the operculum were measured. The results show that thermal pollution has a negative effect on the maturity of the snail.