Fatty acid profiles of larval nematodes (stage‐ 4 Sulcascaris sp.), of tissue from their intermediate host (calico scallops, Argopecten gibbus), and of the host capsule that surrounds the larvae were prepared in an attempt to identify infected scallops. Nematode tissue showed lower ratios of C14:O/C14:1, C16:0/C16:1 and C18:0/C18:1 than did scallop tissue. The nematodes contained relatively less C16:0 and more C18:2 than did scallops. Fatty acids shorter than C14:0 were found in small amounts in both organisms. Fatty acid profiles of capsules differed little from those of normal scallop tissue.