Mammalian mitochondria, as well as rat, plant and Caenorhabditis elegans mitochondria, possess an ATP-sensitive K+ channel (mitoK(ATP)) that has been pharmacologically characterised. Opening of mitoK(ATP) and the subsequent K+ entry into the matrix was shown to have three effects on mitochondria physiology: (i) an increase in matrix volume (swelling), (ii) an acceleration of respiration, and (iii) an increase in reactive oxygen species (ROS) production. These effects on mitochondria bioenergetics have been shown to be part of distinct intracellular signalling pathways, to protect against cell death and to modulate gene transcription. To date, such a channel or its activity has not been described in trypanosomatids. In the present study, we show pharmacological evidence for the presence of a mitoK(ATP) in trypanosomatids. Cells were incubated in a hypotonic medium followed by mild detergent exposure to isolate mitoplasts from Trypanosoma cruzi and Crithidia fasciculata. Mitoplasts swelled when incubated in KCl medium due to respiration-driven K+ entry into the matrix. Swelling was sensitive to the presence of ATP when the mitoplast suspension was incubated in K+ -containing, but not in K+ -free, medium. The ATP inhibition of swelling was reversed by the mitoK(ATP) agonist diazoxide and the diazoxide-induced swelling was inhibited by the mitoK(ATP) blockers 5-hydroxydecanoate (5HD) or glibenclamide. Similar to mammalian and rat mitochondria, trypanosomatid mitoK(ATP) activity was modulated by the general protein kinase C (PKC) agonist phorbol 12-myristate 13-acetate (PMA) and antagonist chelerythrine. As expected, the potassium ionophore valinomycin could also reverse the ATP-inhibited state but this reversal was not sensitive to 5HD or glibenclamide. Dose response curves for ATP, diazoxide and 5HD are presented. These results provide strong evidence for the presence of an ATP-sensitive K+ in trypanosomatid mitochondria.