In order to determine the possibility of infecting mosquitoes with P. juxtanucleare two series of experiments were made. In the first series 158 Aëdes aegypti and 152 Culex quinquefasciatus were examined with negative results. In the second experiments 15 Aëdes lepidus and 80 C. quinquefasciatus were used. All Aëdes lepidus were negative. Theree Culex were found to have become experimentally infected. However, no infection was produced in chickens which were inoculated with salivary glands or stomach removed form the three mosquitoes. C. quinquefasciatus is not considered to be the common vector of P. juxtanucleare because of the very low infection index presented. The results, including those of the unsuccessful chicken inoculations, are briefly discussed.