Using a new source of data, we estimate the probability of apprehensio
n among Mexican migrants attempting to cross into the United States wi
thout documents. Over the period 1965-1989 we found an average apprehe
nsion probability of .35, confirming earlier estimates. We then applie
d annual probabilities to estimate the gross volume of undocumented Me
xican migration and adjusted these figures to derive estimates of the
net undocumented inflow.