A characterization of Simpson's categories of relict species allows discrimination between relictual coevolved associations and associations due to a sequence of host-switching or colonization that coincidentally matches the host group phylogeny. Relictual groups are recognized by (a) highly speciose sister groups and (b) biogeographic distributions indicating great age. Numerical relicts are the relatively few surviving members of a once-speciose group, such as crocodilians. In the absence of a fossil record, past diversity can be inferred from the degree of ecological diversity among surviving members of the clade. Coevolved numerical relicts show spotty distribution among ecologically diverse members of an inclusive host clade. Phylogenetic relicts are “living fossils,” members of old groups that have persisted relatively unchanged for a long period of time, such as horse-shoe crabs. Coevolved phylogenetic relicts are either restricted to host groups that are themselves phylogenetic relicts, or inhabit members of a non-relictual host group that share a common plesiomorphic ecology. Three examples drawn from flatworm parasites and their vertebrate hosts are discussed.