Fitobentos is a biological water quality element of streams and rivers in Croatia, represented in the assessment by diatoms. The index used in the methodology is the Trophic Diatom Index of the Republic of Croatia (TIDHR). The database of phytobenthos monitoring for the period 2010- 2020 includes a total of 581 monitoring stations and contains 1169 data on the composition and relative abundance of diatoms. Of the total 959 diatom species in the operational list, the indicator values and weights of the TIDHR indicate 545 species. The number of species for which indicator values and weights are not available is 415, and the number of species represented in the sample with more than 60 frustules and no indicator values and weights is 26. Consistent with the main objectives of the study, the relative abundances of the 26 species in the samples were correlated with environmental factors. Their distribution, ecological characteristics, and the possibility of introducing new indicator values and weights were indicated. For all calculated OEKs, the correlation shows the response to environmental factors. Although no significant changes in ecological status were found for the whole data set, this study contributes to new knowledge about the indicator system of the Trophic Diatom Index of the Republic of Croatia and indicates what kind of further research and statistical processing should be carried out to improve the indicator system.