This paper is about the species Heteranthera reniformis Ruiz & Pavon found in the rice fields in Kocani-Štip. It is a new adventive species for the flora in Republic of Macedonia. In the text the morphological description of the new weed from the family Pontederiaceae is given. This species Heteranthera reniformis Ruiz & Pavon for the first time is found in July 2004 around Kocani, but less spread in rice fields. In August 2005 this weed is found again in locality between village Cesinovo and village Spancevo left and right from the road. In the rice fields the weed is prevalent on surface about 600 ha and occupied 60-70% from the rice field
