Populations of the elongate hemlock scale, Fiorinia externa Ferris (Homoptera: Diaspididae) are to some extent self-regulated on eastern hemlock, Tsuga canadensis Carriere. Studies conducted during 1976-78 in a hemlock forest in Ridgefield, CT, USA, revealed that survival, development rate, and fecundity of the scale were negatively influenced by its own density for 2 consecutive generations and that the presence and numbers of feeding nymphs adversely affected the quantity (biomass) and quality (nitrogen concentration) of the food available for subsequent consumption. Results from greenhouse and field plot experiments support those from Ridgefield and implicate nitrogen as a critical nutritional element in the self-regulatory mechanism of F. externa on hemlock.