After a spell of teaching at the University of Toronto, he returned to succeed Forrest at Wadham in 1977 when the latter was elected to the Wykeham Professorship of Ancient History at New College.
He was a contributor to the Oxford Classical Dictionary in matters relating to the Hellenistic world and the Roman republic, and wrote the chapter on ‘Rome, the fall of Macedon and the sack of Corinth’ for the 2nd edition of the Cambridge Ancient History (volume 8; 1989).
These (and other papers) represent a sustained argument about the impact of Rome on the Hellenistic world, contesting the previously accepted views of Maurice Holleaux on the nature of Roman expansion and of Polybius' narrative of it.
But ‘Bagnall & Derow’ had a distinctive tone, with its greater emphasis on inscriptions and papyrus texts, and it found favour with many students and teachers who used the two volumes together.
His funeral, attended by a great many colleagues and former students, was held in Wadham College Chapel on 16 December 2006, followed by a memorial service in the Sheldonian Theatre on 28 April 2007.