Here were present a synthesis of the published literature on in situ Carboniferous acritarchs, prasinophyte algae and associated microfossils. The Early Carboniferous had a rich acritarch fossil record, with assemblages documented in both equatorial and high latitude regions. However, the data suggest that there was no relationship between acritarch diversity and palaeolatitude in the Early Carboniferous. Our synthesis reveals a trend of declining acritarch diversity from the Tournaisian to the Visean, and generally very low diversity in the latest Early Carboniferous Serpukhovian and Late Carboniferous. However, fairly diverse acritarch assemblages of up to 13 taxa have been described from the Late Carboniferous of England and South America. They imply that the very low diversity so far documented could result from a lack of research input. However, these assemblages need to be re-examined as they can be based on a limited number of individuals and taxonomic over-splitting is a possibility. Furthermore, the reworking of older assemblages into these Late Carboniferous strata needs to be thoroughly discounted. The data also suggests a trend of declining prasinophyte diversity through the Carboniferous. However, the very low number of taxa typically reported and their grouping under open nomenclature (e.g. Cymatiosphaera spp.) makes this trend of questionable robustness. No clear pattern was observed in the diversity of the Zygnemataceae, although more diverse assemblages could be related to increased terrestrial influence or continental deposition. The drop in acritarch diversity in the Early Carboniferous represents a continuation of a decline that began in the Late Devonian. Although a number of hypotheses have been invoked to account for this Devonian-Carboniferous diversity decline, our synthesis has shown that the phytoplankton from this interval are generally poorly studied, with only a handful of investigations having been completed in stratigraphically continuous sequences. Further work is urgently needed if we are to evaluate these hypotheses and understand this fundamental change in the nature of the ocean's primary producers.