Dogwhelks were collected from 20 sites throughout Sullom Voe and Yell Sound in August 2009 by Aquatic Survey and Monitoring Limited, and transported to FRS Marine Laboratory in Aberdeen for imposex analysis. Dogwhelks of all sizes (juveniles to adults) were collected from five of the sites. Results of the present survey were compared to the ten previous surveys and assessed against the recently developed Oslo and Paris Commission (OSPAR) assessment criteria. The degree of imposex (RPSI and VDSI measurements) in toothed adults from sites within Sullom Voe in 2009 show that these sites continue to be more impacted by TBT from the oil terminal activities than populations at sites in Yell Sound. Throughout Sullom Voe, RPSI and VDSI values in adults (RPSI 0.01-1.08%; VDSI 1.30-3.67) were generally lower than, the values in the 2007 survey. The RPSI and VDSI values of populations outside the Voe (RPSI 0.00-0.67%; VDSI 0.2-3.24), in the well flushed waters of Yell Sound, were much lower than at sites within the Voe. As in previous surveys, the degree of imposex in populations in Yell Sound generally decreases with distance from Sullom Voe. In 2007, four sites within Sullom Voe still showed evidence of females with blocked reproductive tracts, causing sterility in some of the population. For the first time, the 2009 survey shows that at no sites was there evidence of sterility in any of the populations. The RPSI and VDSI measurements at the sites within the Voe show that these populations can continue to reproduce. The levels of RPSI in particular have decreased considerably from 2007, with values having fallen at every site except Billia Skerry (3) and Moss Bank / Grunna Taing (14) where small increases were observed. For the first time since the survey commenced in 1987, substantial decreases in VDSI at both sites within the voe and at the boundary with Yell Sound have been observed, reducing the OSPAR classification of some sites from D to C or B (notably at Voxter Ness (10), such that population-level effects should no longer be being caused by TBT. At sites in Yell Sound there was also a general trend to decreasing VDSI compared to 2007, however two sites, Billia Skerry (3) and Moss Bank / Grunna Taing (14), showed substantial increases in VDSI. In the case of Billia Skerry, this was reflected throughout the smaller juvenile size classes, suggesting that this may be the result of locally recently increased exposure to TBT, but the potential cause of this is unclear. Generally, the survey indicates continued improvement in imposex incidence across the area, likely resulting from reduced inputs from shipping to the area as a result of the IMO TBT ban on large vessels in 2008.