The internal structure of the bacterial virus fd was investigated by ultraviolet dichroism of virus solutions oriented by flow through a small capillary tube. The dichroism was found to be positive for wavelengths longer than 262 mmicro and shorter than 239 mmicro, and negative for the intermediate wavelengths. The magnitude of the effect was at all times small, with dichroic ratios of 1.22 and 0.83 at 280 mmicro and 250 mmicro, respectively. the intuitive interpretation that this was the result of the addition of negative DNA dichroism and positive protein dichroism was confirmed by the application of a simple theory which allowed the calculation from protein and DNA absorption data of a dichroism curve closely approximating the experimental one. The parameters arrived at by this procedure indicate a semiangle of 25 degrees +/-5 degrees for a cone described by the normals to the DNA base planes inside the virus. The protein absorbers tryptophan and probably tyrosine were found to be oriented on the average relatively parallel to the longitudinal axis of the virus.