The carotenoid triplet populations associated with the long-wavelength-emitting chlorophyll forms of photosystem I (PS I) have been investigated in isolated spinach thylakoids by means of fluorescence-detected magnetic resonance in zero field. The spectra collected in the 730-800 nm emission range can be globally fitted assuming the presence of four different carotenoid triplet states coupled to long-wavelength-emitting forms of PS I, having zero-field-splitting parameters \D\ = 0.0359 cm(-1) and \E\ = 0.00371 cm(-1), \D\ = 0.0382 cm(-1) and \E\ = 0.00388 cm(-1), \D\ = 0.0395 cm(-1) and \E\ = 0.00397 cm(-1), and \D\ = 0.0405 cm(-1) and \E\ = 0.00411 cm(-1). On the basis of the triplet-associated fluorescence emission profile, it is suggested that those triplets are associated with light-harvesting complex I, the peripheral antenna complex of PS I.