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We have obtained 21-cm profiles and $I$ or $R$ band surface photometry for a subset of field low surface brightness (LSB) galaxies from the survey presented elsewhere at this meeting by Impey et al. We find that, in comparison to higher surface brightness (HSB) galaxies, a subset of the LSB galaxies are significantly underluminous for their rotation speeds. The LSB galaxies are bluer than the HSB galaxies, which suggests that they have different stellar populations. However, the population differences cannot account for the difference in $M/L$ ratios implied by the deviation from the normal Tully-Fisher relation. The most plausible explanation is that this subset of field LSB galaxies have higher dark-to-luminous matter ratios than do HSB galaxies, a result that is consistent with previous work on LSB dwarf galaxies in the Local Group and in the Virgo and Fornax clusters. Contamination by such underluminous galaxies may account for some of the high peculiar velocities found in previous surveys using the Tully-Fisher relation as a distance indicator.
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