Transition metals affect the viability of organisms and are fundamental and limiting factors at the same time, since they represent nutritionally important yet cytotoxic components in our environment. Both prokaryotes and eukaryotes carry cellular systems to maintain the metal homeostasis.Metallothioneins (MTs) are 6-7 kDa, ubiquitous, cytoplasmic, cysteine rich proteins, which have the capacity of high affinity binding to heavy metal ions. MTs play key role in essential as well as toxic heavy metal metabolism in the organisms. The ciliates tolerate elevated concentrations of metals, which are accumulated, bound to MTs peculiar to these organisms. The MT genes of Tetrahymena tropicalis lahorensis subsp. nov. and Euplotes muscicola SBSrc subsp. nov., tolerating about 500 æM copper in the medium have been described. Gene of copper MT in T. t. lahorensis was amplified using genomic DNA as well as cDNA as template. The cDNA coding for the two isoforms was cloned and sequenced. The nucleotide sequence of one isoform consists of 300, while that of the other has 327 nucleotides. This study provides detailed information about the nucleotide and amino acid sequences of both these isoforms.