The widespread ability of eukaryotic cells to produce thin cytoplasmic sheets or lamellae 100–200 nm thick can give rise to uncertainties in the interpretation of interference reflection microscopy (IRM) images when cell-substratum topography is the key interest. If allowed to spread upon a poly-L-lysine-coated surface, Dictyostelium discoideum amoebae typically form ultrathin lamellae of approximately equal to 100 nm thickness by cytoplasmic retraction. Whereas the cell body is grey, the lamellae appear very dark under IRM optics. These dark areas could be misinterpreted as stemming from a closer cell-substratum apposition beneath the lamellae than the cell body. This ambiguity can be avoided if the technique of total internal reflection aqueous fluorescence (TIRAF) is used in conjunction with a high refractive index glass (n = 1.83) as substratum. Contributions to the image generated by thin cytoplasm and also variable cytoplasmic refractive index are thereby minimized due to the extremely short range of the ‘illuminating’ evanescent wave. From our comparative IRM and TIRAF study of the ultrathin lamellae of Dictyostelium amoebae it is concluded that the cell-glass gap is relatively uniform beneath the entire cell. We briefly discuss the sensitivity of several cell types to TIRAF, the generation of ultrathin lamellae and the nature of the cell-glass gap.

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