The Serpens OB2 association and its thermal "chimney"

UBVRI photometry and MK spectral types have been obtained for a nearly comp lete sample of the massive star population of the Serpens OB2 association. This relatively neglected association is found to contain over 100 OB stars at a distance of 1.9 +/- 0.3 kpc and with a common age of 5 +/- 1 Myr. The se stars illuminate the large H II region S54, and are directly connected t o the unusual thermal "chimney" detected by Muller et al. Evidence is given of continuing star formation within the association, and the relationship between the association stars and the chimney is discussed.