Strengthening of framed reinforced concrete structures by cast-in-place reinforced concrete infills is commonly used in practice. The objective of this study is to investigate the behavior of such infilled frames under seismic loads. For this purpose, 14 two-story, one-bay infilled frames are tested under reversed cyclic loading simulating seismic action. The variables investigated are thc effect of type of infill reinforcement, thc connection between the frame and thc infill, and the flexural capacity of columns. Test results are evaluated to estimate the effects of infill on stiffness, strength, energy dissipation, lateral drift, and ductility. The feasibility of different analytical methods is also investigated. The results obtained using such analytical methods are compared with the experimental observations. Using the test results. a simple dynamic evaluation is made to predict the dynamic behavior of infilled frames under seismic action.