Diatomaceous soils frequently occur in lacustrine and marine deposits. They are usually thought of as one type of material with unusual geotechnical properties characterised by high Atterberg limits, natural water content, compressibility as well as high shear strength. The combination of high liquid limits and high friction angles appears to be a contradiction in the framework of classical soil mechanics, but studies showing relations between their unusual geotechnical properties and micromechanical features of fossil diatoms may provide the key to solving this puzzle. This study considers two diatom species, Aulacoseira granulata and Coscinodiscus centralis. The study identifies the effects of their microscopic features on the macroscopic behaviour of soils. The study involves first, tests of Atterberg limits, and then oedometric and triaxial tests on samples with different proportions of the two species. Results show that the microscopic shapes of the diatoms have observable effects on the macroscopic responses of soils.