In the past years, in situ microscopy has been demonstrated as a technique for monitoring the concentration and morphology of moving microparticles in agitated suspensions. However, up until now, this technique can only achieve a high resolution if a certain manual or automated effort is established for continuous precise focusing. Therefore, the application of in situ microscopes (ISMs) as sensors is inhibited in the cases where unattended operation is required. Here, we demonstrate a high-resolution ISM which, unlike others, is built as an entirely rigid construction, requiring no adjustments at all. This ISM is based on a specially designed water immersion objective with numerical aperture = 0.75 and a working distance of 15 μm. The objective can be built exclusively from off-the-shelf parts and the front surface directly interfaces with the moving suspension. We show various applications of the system and demonstrate the imaging performance with submicron resolution within moving suspensions of microorganisms.