When in Rome is a detective novel by Ngaio Marsh; it is the twenty-sixth novel to feature Roderick Alleyn, and was first published in 1970. The novel takes place in Rome, and concerns a number of murders among a group of tourists visiting the city; much of the action takes place in the "Basilica di San Tommaso", which bears some resemblance to the Basilica of San Clemente, which the author visited 'when in Rome' on an Italian holiday in Summer 1968.