25 samples of the clay pit Frielingen (Upper Hauterivian, North-West German Basin) were investigated concerning their assemblage of calcareous dinoflagellate cysts (3196 cysts altogether). Proportions of taxa, character traits of Obliquipithonelloideae, and the results of some statistical approaches were investigated throughout a succession of the Simbirskites discofalctus zone, which has already been well studied concerning both sedimentological and palaeontological features. The investigation revealed that the general transgressive trend during that time interval, that was already known (Haq et al. 1987, Lutat 1996, Mutterlose 1991) must be modified for the Frielingen area. The thick dark bed 118 seems to comprise a regressive time interval with considerably higher sea surface temperatures. The general sea surface temperature could be reconstructed as well: it shows three intervals of general cooling, interupted by intervals with rising sea surface temperatures. One distintcly warmer interval, in which bed 118 was deposited, represents a dark bed, that is commonly interpreted as being indicative for cooler surface waters (Mutterlose and Ruffell 1999). It is therefore necessary that the interpretation of dark-light-rhythms of Northern Germany concerning the palaeoenvironment is not to strictly simplified. Factors other than sea surface temperature seem to be important as well. The species Pirumella edgarii, as was found out, seems to favour time intervals with higher sediment input. A possibly newly discovered species (Orthopithonelloidea incertae sedis) might be indicative for rather similar conditions.