A preliminary study of the atmospheric extinction at the Bosque Alegre Field Station yields the following results: C₁ = 0.110, C₂ = -0.030; C₃ = 0.271, C₄ = 0.020; C₅ = 0.200, C₆ = 0.000. There is no evidence of any major seasonal variations in the coefficients. With the presently available photoelectric equipment, the magnitudes and colors of the UBV standards are duplicated within ± 0.01 of a magnitude, in the mean.