  We reduced the Hubble Space Telescope Space Telescope Imaging Spectrograph
images of the gamma-ray burst GRB 990123 that were obtained on 8-9 February
1999 and find V_0 = 25.36 +/- 0.10, which corresponds to a flux of 0.258 +/-
0.023 micro-Jy for the optical transient 16.644 days after the burst's peak.
The probable host galaxy has V_0 = 24.25 +/- 0.07 (= 0.716 +/- 0.046 micro-Jy)
and the optical transient is located 0.65 arcseconds (= 5.5 kpc) south of the
galaxy's nucleus. We fit and subtracted a scaled point-spread function to the
optical transient and found evidence for three bright knots situated within 0.5
arcseconds (= 4.3 kpc) of the optical transient. Each knot has V_0 ~ 28.1 +/-
0.3, a rest-frame V-band luminosity of between approximately 5e8 L_Sun and 8e8
L_Sun, and a star-formation rate of at least 0.1-0.2 Solar masses per year. The
knots are centrally concentrated with full-width at half-maximum of
approximately 0.17 arcseconds (= 1.5 kpc). Their sizes and luminosities are
consistent with their being star-forming regions. The optical transient is
located 0.15 arcseconds (= 1.3 kpc) southeast of the centre of one of these
knots.
