The asymmetric male and female genitalia of the spider Metagonia mariguitarensis (González-Sponga) are described, based on semi-thin serial sections and SEM photographs, and several male genital and non-genital characters are measured and analysed statistically. Left male genital bulbs are significantly larger than right bulbs while right palpal tibiae are significantly stronger than left tibiae, suggesting a functional segregation into a more predominantly displaying right palp and a more predominantly sperm-transferring left palp. Despite several structural differences between right and left male palps, however, there is no indication of qualitative differences: both bulbs transfer sperm, and both palps are provided with a full set of muscles. The female internal genitalia are provided with an unpaired receptacle that is connected to the uterus externus by a wide and complicated duct and to the valve separating uterus externus from uterus internus by a short narrow duct. This resembles entelegyne genitalia, but the functional details remain unknown. The evolution of asymmetric genitalia is discussed in a wider framework, with the conclusion that the causes of asymmetry in spiders may be different from those in other arthropods such as insects and copepods.