As a part of the ongoing biodiversity survey of aseptate gregarines in earthworms, samplings were carried out in the East part of the river Ganges of the Murshidabad district. The survey revealed the occurrence of a new species of Monocystis Stein, 1848, namely M. elongatum n. sp. from the earthworm host Perionyx excavatus Perrier, 1872. The gamonts of M. elongatum n. sp. are solitary, elongated and curved at the anterior end and gradually tapering towards the posterior end. The mucron is distinct. Gamonts measure 265.0-346.0 mu m (310.0 +/- 2.6, n = 36) in length and 46.0-64.3 mu m (52.0 +/- 3.1, n = 36) in width. The gametocysts are unique shaped with two equal sized gametocytes, and measure 74.0-98.3 mu m (82.0 +/- 1.9, n = 36) in length and 56.0-82.0 mu m (72.5 +/- 2.7, n = 36) in width. The oocysts are bi-conical, measuring 9.7-15.3 mu m (12.3 +/- 3.1, n = 36) in length and 6.8-10.3 mu m (9.4 +/- 1.1, 36, n = 36) in width.