This experiment investigated the scalp distribution of visual search negativities during a letter search task combined with selection by color. Event-related potentials (ERPs) were recorded from 12 undergraduate subjects when they were required to detect the target letters defined by shape and color. Display load was manipulated by the number of letters presented in the test display, while memory sets always consisted of two alphabets. The results show that a larger display load caused a negative shift of ERPs between 400-650 ms latency range, which was dominant over frontal and central recording site. The time course and the distribution of the visual search negativities were different from those in other studies where subjects were required a simple letter search, and the earlier (about 300 ms) and occipital and posterior temporal effects of display load on ERPs were observed. These differences among experiments suggest that two different systems are involved in a visual search process, and that the functional organization of these systems can change according to the task requirement.