Coelopa frigida is a species of seaweed fly or kelp fly. It is the most widely distributed species of seaweed fly. It can be found on most shorelines in the temperate Northern Hemisphere. Other species of seaweed flies include Coelopa nebularum and Coelopa pilipes. C. frigida feeds primarily on seaweed, and groups of C. frigida flies tend to populate near bodies of water. Climate change has led to an increase in C. frigida blooms along shores, which creates a pest problem for human beach-goers. C. frigida is also an important organism for the study of sexual selection, particularly female choice, which is influenced by genetics.

Coelopa pilipes is a common European species of kelp fly. It was described by A. H. Haliday in 1838. Their appearance differs greatly from that of other Coelopa flies.