This study reports on a manipulative experiment demonstrating that natural blooms of Ulvaria obscura (Kutzing) Gayral, a relatively understudied and easily misidentified green alga, can cause the loss of eelgrass shoots in a subtidal meadow. A total of 20 1m 2 quadrats were set at the inshore edge of an eelgrass (Zostera marina L.) meadow near Blakely Island, Washington State, USA. Ulvoid algae were removed from experimental quadrats and left in control quadrats. Experimental quadrats lost 12% of their eelgrass shoots over 5 weeks, while control quadrats lost 54% of their eelgrass shoots. Thus, we conclude that natural blooms of U. obscura at the study site reduce eelgrass shoot density.