TITLE: GCN CIRCULAR NUMBER: 21200 SUBJECT: GROND observations of GRB170604A DATE: 17/06/05 11:20:22 GMT FROM: Patricia Schady at MPE/Swift P. Schady (MPE) reports: We observed the field of GRB 170604A (Swift trigger 755867; Page et al., GCN #21191) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPG telescope at ESO La Silla Observatory (Chile). Observations started at 06:15 UT on 05-06-2017, 11.1 hours the GRB trigger. They were performed at an average seeing of 1.3" and at an average airmass of 2. We detect a point source within the 1.4" enhanced Swift/XRT error circle (Page et al., GCN 21196), and consistent with the afterglow position reported by Izzo et al. (GCN #21192). Based on a total exposure of 4.6 min in g'r'i'z' and 4 min in JHK, we estimate preliminary magnitudes (all in AB) of g' = 20.75 +/- 0.08 r' = 20.46 +/- 0.05 i' = 20.35 +/- 0.06 z' = 20.21 +/- 0.08 J = 19.6 +/- 0.1 H = 19.6 +/- 0.3 K > 19.6 Given magnitudes are calibrated against PanSTARRS and 2MASS field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V)=0.03 mag in the direction of the burst (Schlafly & Finkbeiner 2011). We acknowledge the excellent support provided by Simona Ciceri in obtaining these data.