There are several sites in the world where diatom microfossils have been detected in the soil, these natural deposits have singular physical and mechanical properties. However, the specialized literature on the topic is very scarce. To evaluate the influence of microfossils on the cyclic response of clayey soils, this paper presents the experimental results of two series of cyclic simple shear tests using an artificially prepared mixture of kaolin and diatom microfossils (K+D). Tests were carried out on normally consolidated and overconsolidated samples. Based on the available literature and the obtained results, an attempt was made to explain the influence of diatom microfossils on the cyclic behavior of fine-grained soils.