Experimental evidence is presented showing that excitons in circular antenna complexes from photosynthetic bacteria are dynamically self trapped in about 200 fs by coupling to nuclear vibrations. The induced deformation covers similar to 20% of the complex circumference at low temperature. This self trapping, the first of its kind observed in biological systems, results in a broad fluorescence spectrum and considerably improves energy resonance between heterogeneous antenna complexes. Exciton self trapping may thus be a part of nature's strategy, increasing the speed and efficiency of energy transfer in photosynthesis.