CuInS2 films grown epitaxially on Si (111) substrates exhibit a number of extended defects such as stacking faults, coherent and incoherent twin boundaries, and micro twins. These defects are partly induced by the interface strain due to the lattice of thermal mismatch between Si and CuInS2. The appearance and density of the planar defects depend on film stoichiometry: In Cu-rich films, coherent twin boundaries form on ((112) over bar) surfaces but are absent on (<(11)over bar>2). The density of incoherent twin boundaries and dislocation lines is small. Film growth appears to apply a step flow mechanism on [110] steps with the result of defect free regions laterally extending up to 1.3 mu m. Incoherent twin boundaries and stacking faults are concentrated near the interface region. For Cu-poor films, the density of stacking faults and dislocation lines on the (112) plane is much higher. Stacking fault and twin formation is also found on (<(11)over bar>2) planes. The film volume free of planar defects is smaller. This difference between Cu-rich and Cu-poor CuInS2 films is discussed in relation to the different point defects induced by the stoichiometry deviation. A model for the formation of stacking faults is proposed.