We used a highly collimated monochromatic X-ray beam to measure the diffraction from a single monolayer and from odd numbers (up to 11) of layers of manganese stearate (MnSt2). The molecules were deposited in layers by the Langmuir-Blodgett technique. For the larger numbers of layers we can distinguish that the continuous diffraction pattern consists of several orders of (00l) Bragg peaks and "subsidiary" maxima which arise between these. Below the (001) peak some subsidiaries are larger than the largest Bragg peak. Qualitative explanations of the results are given. The diffraction patterns are quantitatively explained by calculations of the interference of X-rays reflected from layers whose electron density corresponds to the structure of MnSt2. Good fits to the data are obtained with parameters of the layers that are close to a priori estimates. The results also show that our silicon substrates were smoother for our films than glass microscope slides were. © 1980.