Clonal cultures show that typical specimens of the arcellacean Difflugia tricuspis Carter are capable of producing descendants with tests made either of idiosomes, or a mixture of idiosomes and xenosomes, or of xenosomes only. The same cultures also demonstrate that individuals of this group are capable of producing an impressive array of different morphotypes. Using this group as a practical example, we elaborate on the concept of interval of variability as revealed by clonal cultures of largely uniparental organisms such as thecamoebians and foraminifera. We also discuss the practical application of this concept to the solution of complicated taxonomic problems. Previous authors have considered at least 98 forms, identical to our morphotypes, as independent taxa. As they all fall within the interval of variability delimited by our cultures, we believe that they belong to the single species D. tricuspis Carter.