Eleven endolithic forms were distinguished from skeletal fragments collected near the Gulf Islands and from banks in the Strait of Juan de Fuca, British Columbia. The vast majority of studies dealing with endoliths are from tropical regions; however, the abundance of boring organisms described herein demonstrates their importance as sedimentological agents in mid-latitudes as well.Although some, if not most, of the borings occurred after the substrate organism was dead, the order of infestation was indeterminant. Fungal borings are densely distributed when in close proximity to algae. Although the nature of this biological association remains undefined, this relationship suggests that fungi prefer utilizing algae as an energy source. The observed decreased infestation of algal endoliths in shallow environs (less than 30 m) compared with that in deeper water might be attributed to less exposure in a high energy regime or to a freshness factor of the shells. Algal endoliths were observed in several shell substrates obtained from below the photic zone (defined as 45 m on the basis of the deepest occurrence of encrusting red algae). This distribution of endolithic microphytes indicates that any bathymetric zonation that may have been initially present was disturbed by the lateral transport of substrate particles. Furthermore, this suggests that algal borings should not be used independently of other environmental information to determine paleobathymetry.