A new strain of planktonic heliozoans (ZI172) belonging to the genus Microheliella (the sister group of Cryptista in Diapho-retickes), closely related to the only one known strain of Microheliella maris (CCAP 1945/1), was studied with light microscopy and SSU rRNA gene sequencing. Morphometric data obtained from 127 cells and based on 254 measurements showed that this strain represents the smallest heliozoan (1.66–3.42 µm, av. 2.56 µm) in diameter known to date and one of the smallest free-living eukaryotes. We also did morphometry for strain CCAP 1945/1. Its cell body size is 3.20–6.47 µm (av. 4.15 µm; n=141; m=282). The secondary structures of hairpin 15 of the SSU rRNA molecules were reconstructed for ZI172 and CCAP 1945/1 and they were compared The possible biochemical explanation for the smaller size of the ZI172 strain, which is smaller than the CCAP 1945/1 strain, is discussed, including all published electron micrographs of CCAP 1945/1. The necessary taxonomic work is also carried out. The diagnosis of Microheliella maris is amended and the new infraspecific clade Erebor is described to include ZI172. The measurements and systematics of the enigmatic heliozoan ‘Raphidiophrys’ magna O’Donoghue 1922 (non 1921; the biggest known heliozoan) are also discussed and it is transferred to the new genus Berkeleyaesol.