The influence of the holoparasite Cuscuta campestris Yuncker on the growth and photosynthesis of Mikania micrantha H.B.K. was studied. The results indicate that C. campestris infection significantly reduced the light use efficiency and light saturation point of the host. It significantly reduced the net photosynthetic rate (P(n)) of the 1st and 8th mature leaves of M. micrantha at light saturation point, the apparent quantum yield of the 1st mature leaves, the carboxylation efficiency and CO(2) saturated P(n) of the 8th mature leaves, but increased the light compensation point of the 1st mature leaves. Diurnally, it significantly reduced P(n) between 08.00 h and 16.00 h and stomatal conductance and transpiration from 10.00 h to 16.00 h for the 8th mature leaves. Moreover, the significantly adverse effects of C. campestris infection on P(n) were observed 18 d after parasitization (DAP) for the 4th, 8th and 12th, and 25 DAP for the 1st mature leaves of M. micrantha, and they became greater with infection time. The infection also significantly reduced the number of leaves, leaf area, stem length, and biomass, and prevented flowering of M. micrantha in the growing season, and caused almost complete death of the aerial parts of the host about 70 DAP, but the uninfected plants grew and developed normally. Furthermore, the total biomass of the infected host and the parasite was significantly less than that of the uninfected plants. Therefore, besides resource capture by C. campestris, the reduced growth of the infected plants must also be due to the negative effects of the parasite on host photosynthesis.