Predator-prey-relationships are often investigated and described with respect to consumption on prey as a lethal (=killing) effect. However, knowledge on the role of non-lethal predator effects that may change prey behavior, such as predation risk effects on prey reproduction, is scarce. It is assumed that these risk-effects are likely to contribute strongly to the total effect of predators. A reduction in the reproductive output of the common periwinkle Littorina littorea while exposed to the green crab Carcinus maenas was already demonstrated in the laboratory. This risk-effect was ought to be shown in the field by the following experiments in the rocky intertidal shore of Helgoland. In prior to the field experiment (two runs, 7 days each) I designed two factorial laboratory experiments (10 days each) in order to analyze the influence of (1) tides and (2) snail-density on the reproductive output of L. littorea in the presence and in the absence of C. maenas. Besides number of eggs, I measured algal consumption (Fucus serratus) and shell thickness. In the density-experiment L. littorea reduced eggs by 20 % in the presence of C. maenas. No influence of tides or snail-density on the risk-effect was found. Based on this fact, I did expect the risk-effect to appear in the field. In the field I found that snails produced 38,4% less eggs in the presence of C. maenas. An increase in the number of eggs appeared from March to May, afterwards the number of eggs decreased within 5 days by 78%. Fucus consumption was only significantly reduced in the density-experiment (2) while exposed to a green crab. Shell-growth did not change between all treatments. A correlation (40%) of Fucus consumption and number of eggs was only demonstrated for the density-experiment. Since tides and snail-density did not influence the risk-effect of C. maenas on the reproductive output of L. littorea, I conclude that the effect is likely to appear without temporal or spatial restrictions in the area where both species coexist. The findings emphasize the importance of risk- effects in comparison with lethal effects.