APB (DL-2-amino-4-phosphonobutyric acid) has been found to affect the retinal processing of many vertebrate species as evidenced by the suppression of the b-wave component of the electroretinogram (ERG). The present study examined the effects of APB on the cone contributions to the ERG response of adult zebrafish (Danio rerio). ERG responses were obtained from light-adapted adult zebrafish following intravitreal injection of either saline alone or saline with various concentrations of APB ranging from 10 μm to 500 μM. Visual stimuli were 200-ms flashes of various wavelengths and irradiances. Spectral sensitivity functions were calculated from the irradiance versus response amplitude functions of the a-, b-, and d-wave components of the ERG response. Saline had no effects on the ERG response. However, APB had differential effects on the sensitivity of the b- and d-wave components. The effects of APB on the b-wave component were most apparent in the ultraviolet and short-wavelength portions (320–440 nm) of the spectral sensitivity function, although the b-wave was not completely eliminated at these wavelengths. APB-treated subjects were found to possess the same cone mechanisms (L-M and M-S) in the middle- and long-wavelength areas of the spectrum as saline injected subjects, although absolute sensitivity was lower for the APB-injected subjects. Spectral sensitivity based on the d-wave response was affected by APB but only in the short-wavelength region. All results appear to be independent of the APB dose. These results support the notion that glutamate receptors play a specific role in zebrafish visual processing. In addition, the effects of APB support recent anatomical evidence that the zebrafish retina may possess different types of glutamate receptors.