Boundary formations (lomasomes) have been observed between the plasmalemma and wall of mesophyll cells in Puccinia graminis and P. recondita infected wheat plants. They were present in host cells from both resistant- and susceptible-type infection centers, and were especially frequent and massive in plants grown at low (65–70 °F) temperatures. They appeared to occur in response to the presence of the pathogen, but did not seem to be associated with either host resistance or susceptibility. The dark-staining masses present in the lomasomes were often dense and extremely irregular in shape, but some of the dark material occasionally appeared vesicular or tubular. The suggestion is made that the lomasomes observed in the present investigation may be associated with infection-induced alterations in the mesophyll cell wall.