Hiatus concretions are arly-diagenetic sedimentary bodies, which in their history of formation, underwent exhumation during sedimentation break and/or erosion of sea-floor. Then they were colonized by various encrusters and borers, before they were buried again. Within Middle Jurassic (Bajocian through Bathonian) clays, hiatus concretions occur in four localities in the Cracow-Częstochowa Upland (south and central Poland): Ogrodzieniec, Bugaj, Mokrsko and Krzyworzeka. Preliminary data about their mineralogy, organic geochemistry and palaeoecology is given. Mineralogical analyses showed, that except dominant high Mg-calcite (up to 87.10% of total carbonates), concretions possess minor amounts of such compounds as quartz, clay minerals (kaolinite and illite), pyrite, siderite or Mg-kutnahorite. However, high differences are observed between total organic carbon contents of concretions (TOC=0.46%) and surrounded clays (TOC = 2.16%), but their molecular composition is similar. Most of identified biomarkers are of terrestrial origin. In clay lithology, the hiatus concretions are the only sedimentary bodies that clearly mark the sedimentation pauses (hiatuses). In the investigated area, they form more or less continuous horizons. The concretions are calcitic, possessing various encrusting fauna such as bryozoans, oysters and oyster-like bivalves, serpulids, solitary corals and foraminifers. They often post-date the borings, belonging to such ichnogenera as Gastrochaenolites, Trypanites and Entobia. Some of the nodules show distinct transition from firmground (characterized by the presence of Glossifungites ichnocoenosis) to hardground (presence of Trypanites ichnocoenosis), pointing to the fact, that they formed at, or very close to, the sediment-water interface. Some concretions, like those from Bugaj and Mokrsko, are characterized by their high diversity of hard bottom communities. Those concretions are also irregular in shape; others, like those from Ogrodzieniec and Krzyworzeka, possess lower diversity. The concretions from Krzyworzeka are especially dominated by the borers, while the encrusters are sparse and not diverse. Those nodules that are wide and flat tend to be more bored on one surface only, while those that are more roundish, are bored on both sides more evenly. The degree of diversity is probably correlated with physical disturbance, causing the rolling and overturning of the exhumed concretions in the littoral zone. On the other hand, the overall diversity could be lowered due to destructive abrasion either of the shallowest borings, echinoid/gastropod scratch marks and shells of nestling bivalves, as well as more delicate epilithozoans.