Mayfly eggs possess 4 main morphological features which may be used for taxonomic purposes: chorionic sculpturing, polar cap(s), accessory attachment structures, and micropylar devices. A new and simple technique is described for observing these structures. A detailed morphological description of the egg is presented, including terminology for the parts of the micropylar device. The eggs were found useful for generic or specific determinations for the Heptageniidae, Leptophlebiidae, Caenidae, Ephemeridae, and Polymitarcidae; and of doubtful value for the Ephemerellidae, Tricorythidae, and Potamanthidae. Their value in taxonomy is not now assessible for the Siphlonuridae and Ametropodidae.