Triplet-triplet energy transfer among peridinin molecules in peridinin-chlorophyll-protein (PCP) complexes from two different dinoflagellate species is quantitatively evaluated by the use of the optically detected magnetic resonance technique supplemented by triplet-state decay rate measurements. The two complexes are related by a dimer-monomer relationship in the sense that the four peridinins-one chlorophyll cluster of the Heterocapsa pygmeae PCP (H-PCP) is a copy of the two clusters contained in the larger protein Amphidinium carterae PCP (A-PCP), which are related by a local 2-fold symmetry axis. The system is conveniently discussed as a multistate Frenkel exciton system performing stochastic jumps from one exciton state to another. The rate of triplet-triplet energy exchange among peridinin molecules is obtained through the simulation of the spectral line shapes of optically detected magnetic resonance in zero field. Measurements were collected in a range of temperatures between 2 and 40 K and supplemented by the determination of the decay rate of the triplet to ground state in the same temperature range. The latter is of the order of 0.005 MHz and is relevant in determining the line shapes, especially at the lowest temperatures where the exchange rates are low. Exchange rates within the four-member peridinin cluster of H-PCP exactly match those within each of the two clusters of A-PCP and span a range from 0 to 20 MHz for the uphill rate and from 10 to 300 MHz for the downhill rate, corresponding to an energy difference of 41 cm(-1). The intercluster exchange rate in A-PCP goes from 0 to 12 MHz uphill and from 0.05 to 30 MHz downhill, with an energy difference of only 12.5 cm(-1). The kinetics conform to phonon-stimulated radiationless transition theory, and coupling parameters and reorganization energies are evaluated and discussed.