We present deep narrow-band ( \lambda = 5390 Å , \Delta \lambda = 77 Å ) and multi-color observations of the Hubble Deep Field and the Hawaii Deep Field SSA22 obtained with the LRIS instrument at the Keck II 10-m telescope .
It is shown that there is a substantial population of galaxies at z \sim 3.4 which can be selected by Ly \alpha emission .
Comparison with color-selected samples shows that the samples selected with these different criteria have substantial , but not complete overlap , and that there is a comparable surface density in the two selected populations .
The emission-line selected samples include objects with strong Ly \alpha , and which are significant contributers to the integrated star formation at these epochs .
For a Salpeter IMF we estimate a minimum star formation rate of 0.01 M _ { \odot } Mpc ^ { -3 } yr ^ { -1 } at z = 3.4 for H _ { 0 } = 65 km s ^ { -1 } Mpc ^ { -1 } and q _ { 0 } = 0.5 in the Ly \alpha -selected objects , though the value could be substantially higher if there is significant extinction .