The Fecundity of female Bursera simaruba (L.) Sarg. (Burseraceae) trees was found to be negatively correlated with their degree of coverage by lianas in the deciduous forests of Santa Rosa National Park Guanacaste Province Costa Rica. Experimental reduction of the liana of heavily overgrown trees resulted in an increase in fruit production. The finding that lianas are detrimental to the fecundity of their hosts coupled with the fact that lianas require physical contact with trees as a consequence of their growth form suggests that lianas should be viewed as structural parasites of the trees that support them.