Shell ultra-structure and morphometrical variability of Nebela tenella were investigated using scanning electron microscopy (SEM). N. tenella was isolated from two widely separated populations, one from Switzerland and another from Canada. The shell’s structural elements were similar to those of the other nebelids, but N. tenella has a characteristic peculiarity – always present depressions on the shell surface, which makes an uneven outline of the shell. Moreover, light microscopy and SEM study showed that the collar of the N. tenella represents a turned-over continuation of the neck, which is covered by the same idiosomes as on the shell body. The biometrical analysis showed that the majority of the basic characters of N. tenella vary moderately and give continuous series of transition forms. According to the shell depth and the ratio depth/width both populations were significantly different of each other. Variation coefficients showed that the variability of the characters differs in both populations and the Swiss population is more stable than the Canadian one. All new obtained data for N. tenella raise the question whether the shell’s size, cross section and ultramorphology are reliable enough as characters for the differentiation of N. tenella and N. griseola, and whether they are two distinct species or should just be considered as ecophenotypic variation within one species?