A 1978 by-election was held for the House of Commons parliamentary constituency of Pontefract and Castleford on 26 October 1978 following the death of Labour Member of Parliament (MP) Joseph Harper on 24 June. It was one of two UK parliamentary by-elections held on that day.
The result was a Labour hold. The party's majority was reduced from the 54.2% at the October 1974 general election to a still comfortable 38.5%. Both Labour and Liberal Party candidates saw their votes fall, the latter's share of the vote being nearly halved.
The winner, Geoffrey Lofthouse, served as the constituency's MP until his retirement at the 1997 general election.
| Party | Candidate | Votes | % | ±% | |
|---|---|---|---|---|---|
| Labour | Geoffrey Lofthouse | 19,508 | 65.8 | −4.6 | |
| Conservative | Hugo Page | 8,080 | 27.3 | +11.1 | |
| Liberal | Leslie Marsh | 2,054 | 6.9 | −5.4 | |
| Majority | 11,428 | 38.5 | −15.7 | ||
| Turnout | 29,642 | ||||
| Labour hold | Swing | -7.9 | |||
