In the present work, in-situ micromechanical tensile tests were carried out for hexagonal close-packed zinc and magnesium. Due to their technological relevance, e.g. for rechargeable zinc-based batteries and biomedical magnesium implants, it is important to better understand these materials and their specific mechanical properties, since they behave different from cubically crystallized metals. In cubic crystals all axes are of equal length, whereas in hexagonal crystals a = b ≠c applies. This lower symmetry leads to an anisotropy of the mechanical properties, whereby the c/a-ratio is decisive for the deformation mechanism. The limited number of slip systems that can be activated in hcp materials can support the occurrence of twinning. Another important factor is the sample dimension, as a mechanical size-effect can occur in the micron regime, which in turn influences the deformation behaviour. High purity (99.999%) single crystals were used for the experiment. These were processed in the focused ion beam (FIB) microscope to produce micro-tensile specimens, with three thicknesses spanning from 2 μm to 6 μm. Subsequently, tensile testing was carried out in situ in a scanning electron microscope (SEM). The samples were thereby loaded in such a way that they were extended along the c-axis, i.e. in the [0001] direction. During the experiment, the force and displacement were measured, from which the stress-strain curves were calculated. Furthermore, continuous videos of the specimen deformation were recorded. Afterwards, the results were evaluated and discussed. The size effect on strength could be confirmed in both materials, while different deformation mechanisms, namely as twinning in zinc opposed to dislocation slip in magnesium, could be evidenced.