Summary All species of the genus Quadrulella recorded in Bulgaria are described morphologically and morphometrically and distributional data for them are provided. A new testate amoeba Quadrulella deflandrei sp. nov. isolated from Sphagnum mosses is described. The newly described species resembles Q. symmetrica in its general shape, but differs from it by about twice the size and significantly more and larger shell plates (maximum size 15–19 µm in Q. deflandrei vs. 10–12 µm in Q. symmetrica). On the other hand, Q. deflandrei has a similar size as Q. quadrigera and Q. squtellata but can be easily distinguished from them by the size and arrangement of the plates: in Q. quadrigera shell plates are relatively small and similar in size (8–10 µm), arranged in numerous rows, often mixed with single round or elliptical plates; in Q. scutellata , small quadrangular plates are usually superimposed at the junctions of the larger plates. The analysis of the variation coefficients shows that the population of the newly described species is homogeneous and all characters and ratios measured are weakly to moderate variable (CV range from 4.98% to 9.78%). The shell length, breadth and depth, as well as length/breadth ratio are the most stable characters (CV between 4.98% and 6.79%). Moderate variability is observed for the aperture and breadth/aperture ratio (9.24% and 9.72%, respectively). Our study of many individuals of Q. longicollis gives us evidence to support the view that it is a well-defined and independent species, clearly distinguishable from Q. symmetrica and Q. variabilis. We discuss the high morphological and morphometric variability of Q. symmetrica and the need for molecular studies of European narrow-shelled and tubular forms, very similar to the recently described South African species Q. madibai , to elucidate their taxonomic status. We synonymise Quadrulella symmetrica var. irregularis Penard, in Wailes and Penard, 1911 with Quadrulella longicollis (Taranek, 1882).