Quantum dots (QDs) are recognized as the excellent fluorescence and photochemical materials to be applied in bioimaging, biomedical, and solar cell fields. Biosynthesized QDs (bio-QDs) have attracted attention due to their simple, eco-friendly, and excellent biocompatible traits. Moreover, bio-QDs could not be replaced by chemically fabricated QDs in many fields. Bio-QDs synthesized by different microorganisms have diverse characteristics. In this work, the biosynthesis of QDs by Tetrahymena pyriformis, a typical protozoa in aquatic environments, was achieved for the first time. The synthesized materials by T. pyriformis emitted yellow fluorescence and had an average diameter of 8.27 +/- 0.77nm. Spectral characterization results demonstrated that the synthesized QDs were CdS1-XSeX. Meanwhile, the fluorescence intensities of the synthesized bio-QDs showed a linear relationship with Cd2+ dosage ranging from 20 to 80M. The fluorescence enhancement of the synthesized QDs was highly selective to Cd2+ compared to other metal ions. The bio-QDs were demonstrated to have a great potential to be applied for Cd2+ detection. This work provides valuable information about the transformation of heavy metal ions in protozoan and is useful to accelerate the applications of the synthesized QDs.