Recognising the presence of a parasite and identifying it is a relatively straightforward task for the present day parasitologist. Not so the pursuit of ancient parasites in fossil organisms, a much more difficult proposition. It is probable that most Phanerozoic parasites left no recognizable evidence of their presence; we will never know that they were there. Evidence for infestation may be a palaeopathology, a trace fossil (requiring careful interpretation) or, rarely, the presence of the body fossil of the parasitic(?) organism. Some selected examples from the Mississippian of the British Isles include a zaphrentoid coral on the camerate crinoid Amphoracrinus may have robbed food from the arms; a pit in what appears to be a carefully selected site on the disparid crinoid Synbathocrinus which is also associated with a growth deformity of the cup; and multiple pits in an Amphoracrinus theca are also associated with a deformed cup, but it is more difficult to interpret. Some specimens of the Permian camerate crinoid Neoplatycrinus from Timor have circular grooves or depressions posteriorly, presumably produced by coprophagic/parasitic platyceratid gastropods. From these and other examples it is deduced that sparse infestations of borings or epizoozoic organisms permit a more confident interpretation of organism/organism interactions; dense accumulations, possibly following multiple spat falls, mask such patterns.