The coccal unicellular green algal genus Tetraëdron Kütz. ex Korshikov, which can be easily identified by its typical polygonal shape, is a common member of freshwater plankton and metaphyton, frequently observed in lowland temperate and tropical waters. During the analysis of samples from tropical Lake Kivu (Eastern Africa), we found an interesting "lemon-shaped" alga, which, after observations in light microscope and scanning electron microscope, had been listed as Tetraëdron sp. Isolation in pure culture allowed a deeper study on morphology at different stages of the life cycle and the partial sequencing of the 18S rDNA. The results from the different combined approaches confirmed that it belongs to the species Tetraëdronminimum (A. Braun) Hansg. The unusual "lemon-shaped" forms predominant in Lake Kivu are young stages of the life cycle. This study contributes to the knowledge of the morphological variability, reproduction, and resting stages of T. minimum and discusses the reasons for the dominance of such unusual shape found in Lake Kivu, a lake strongly impacted by human activities as resulted by the large-scale biomanipulation following the introduction of the "Tanganyika sardine," Limnothrissa miodon (Boulenger, 1906), at the end of the 1950s.