With the purpose of identifying relationships between light conditions and biomass of floor vegetation in the understory of a forest, an experimental study was implemented in the Philippines. The study site was selected in a tropical evergreen forest, which was predominated by Pometia pinnata and Celtis luzonica. The forest floor, which was weeded in advance, was covered with vegetation, consisting mainly of pinkball (Calliandra portoricensis), in 13 months after bush cutting. Five plots having pinkball shrubs of different heights were set up in the floor vegetation for the measurement of light and biomass status. Light conditions in the forest were measured with two methods: integrating photometers and hemispherical photographs. The former was exposed for five days in March, 1989. Based on the values of integrated light in each plot, the relative solar radiations were calculated. Using hemispherical photographs, which present areas of canopy gaps, relative diffuse and direct light intensities were calculated. Regarding the relationships among the three different indices of light conditions in five plots, a greater correlation was observed between two indices calculated from the photographs than the other pairs. The relationships of three indices each with the biomass of floor vegetation under different light conditions were not linear. From these results, it is concluded that in the dark understory of a forest, pinkball shrubs grow exponentially in the course of time in different relative growth rates proportionate to the relative light intensity and that the relative growth rate decreases in a more open site. It was also suggested that a greater influence on the growth of pinkball be provided by the penetrating direct light than the diffuse light in the dark understory.