Cerulenin-induced modifications in the fatty acid composition have been used to investigate the influence of acyl lipids on excitation energy distribution in thylakoid membranes of Petunia hybrida by means of 77 K fluorescence spectroscopy. Although cerulenin has no effect on relative contents of chlorophyll and acyl lipids, changes in the fatty acid composition of all thylakoid acyl lipids have been observed. The main cerulenin effect seems to be an increase in linoleic acid at the expense of saturated and monounsaturated C ₁₆- and C ₁₈- fatty acids resulting most likely in an increase in acyl lipid species containing both linoleic and linolenic acid. Low temperature (77 K ) fluorescence kinetics reveal a remarkable decrease in the ratio of the variable divided by the maximal fluorescence of photosystem II (F_2(v)/F_2(M) ), taken as indicator for cerulenin-induced changes in this photosystem. Calculations of the excitation energy distribution terms based on a grouped bipartite model of photosynthesis suggest that a decrease in this ratio is caused by changes in energy transfer probabilities responsible for both, photochemical trapping of photosystem II and energetic cooperativity (grouping) between different photosystem II-light harvesting complex-units. Moreover, changes in the conformation responsible for spillover energy transfer are most likely to occur. Correlations between cerulenin-induced modifications of fatty acid composition and energy distribution support the assumption that excitation energy transfer depends on the structural state of the lipid matrix.