The tarnished plant bug (TPB), Lygus lineolaris (Palisot de Beauvois), is becoming a significant late-season pest in Louisiana cotton. The effects of TPB density and boll age on boll abscission were studied during 1997-1998. TPB adults were caged on bolls classed according to heat units (HU) accumulated. Boll abscission was recorded at 72 hours after infestation (HAI), 7 days after infestation (DAI) and at harvest. Significant differences in boll abscission between TPB infestation levels > 2 TPB/boll and infestation levels < 1 TPB/boll were observed at 72 HAI and 7 DAI. At 72 HAI and 7 DAI, cotton bolls that had accumulated < 265 HU or 11.7 days beyond anthesis had higher boll abscission rates than that for the non-infested control. At harvest, boll abscission rates for the infested bolls remained higher from the non-infested controls until bolls accumulated approximately 300 HU or 13.3 days beyond anthesis.