We present the results of a narrow-band survey of QSO fields at redshifts that place the [ OIII ] ( 5007Å ) emission line in the \frac { \Delta \lambda } { \lambda } \sim 1 % 2.16 \mu m filter .
We have observed 3 square arcminutes and detected one emission line candidate object in the field around PC 1109+4642 .
We discuss the possibilities that this object is a star-forming galaxy at the QSO redshift , z _ { em } = 3.313 or a Seyfert galaxy .
In the former case , we infer a star formation rate of 170M _ { \odot } /yr for this K ^ { \prime } =21.3 object .
The galaxy has a compact but resolved morphology , with a FWHM=0.6 ^ { \prime \prime } , or 4.2kpc at z=3.313 ( H _ { 0 } = 50 km s ^ { -1 } Mpc ^ { -1 } and q _ { 0 } = 0.5 ) .
The comoving density of such objects in QSO environments appears to be 0.0033Mpc ^ { -3 } , marginally lower ( \leq 3 \sigma ) than the density observed for H \alpha -emitters in absorption-line fields at z \sim 2.5 , but similar to the density of Lyman Break Galaxies at z \sim 3 .
If on the other hand , most of the line emission is [ OIII ] from a Seyfert 2 nucleus at z=3.31 , then the high inferred volume density could imply a large evolution in the Seyfert 2 luminosity function from the current epoch .
We find the field containing the object to also contain many faint extended objects in the K ^ { \prime } image , but little significant excess over the expected number-magnitude relation .
We discuss the implication of the emission line being a longer wavelength line at a lower redshift .