•Ancient forest species have a tendency to coexist within distinctive groups.•The coexistence of ancient forest species is an indicator of high species richness.•Groups of coexisting ancient forest species indicate forest biodiversity hotspots.•The deciduous forests are important in determining the primary species composition. Continuity of forest habitats is evaluated by using the indicator value of plants considered as ancient forest species or closed forest species. This is a subjectively defined group, which includes species that prefer old-growth forests. Until now, the fidelity between ancient forest species and its relation to species richness has not been assessed. Analyses were performed using resources of the Polish Vegetation Database, from which we selected relevés containing at least one species from the group of ancient forest species. Subsequently, we examined whether these species demonstrate a tendency to coexist. We established these relations using the phi coefficient of fidelity. We have distinguished eleven groups of coexisting ancient forest species (CAFS groups). Fairly common taxa in the data set constituted the majority of species forming these groups (frequency of >10%). Only one CAFS group found in the dataset was strongly localised geographically. The other groups did not demonstrate specific patterns of distribution and were recorded in relevés throughout the country. The average species richness of forests, in which at least one CAFS group was present, was significantly higher than the average in the forests, where there were no groups of CAFS found. These forests also differed significantly in the total number of ancient forest species and the number of closed forest species. We found that the coexistence of ancient forest species was an indicator of forest communities characterized by a high diversity. In all types of forests identified in the data set, at least one group of coexisting ancient forest species was identified. However, the occurrence of the CAFS groups was not equally frequent in all types of forests. CAFS groups occurred most often in the oak-hornbeam forests, species-rich beech forests and ravine forests, which are zonal types of vegetation. On the other hand, the CAFS groups were rarely found in bog Scots pine woodlands, acidophilous spruce forests, alder carrs and willow-poplar forests of lowland rivers, and these are mostly azonal vegetation types with a limited range. CAFS groups can be used as a trans-regional indicators of forests with a high diversity of species, however, their usefulness may be limited in relation to forests found in oligotrophic habitats or azonal forest systems.