This work reports on the phase diagram study of the dense Kondo system CeB6 by means of magnetization (M) and transverse magnetoresistance (MR) measurements in magnetic fields (B) to 1ST, in the temperature (T) range from 1.8 to 40 K. The zero field resistivity displays Kondo behavior in the paramagnetic P-phase, while two discontinuities at low T were observed at TQ ≈3.5 K and TN ≈ 2.3 K, the boundaries of the antiferro-quadrupolar (AFQ), and antiferromagnetic (AF) phases, respectively. The isothermal transverse MR is large and negative at low T, and it shows a distinct feature when the P-AFQ line of the B-T phase diagram is crossed. Similarly, the M vs. B curves at low temperatures show upward discontinuities at the phase boundary. Both the features in MR and M at the P-AFQ line correlate well in T, revealing the gradual increase of the T for the onset of AFQ ordering with B. No evidence for reentrant behavior could be observed in fields to 1ST. © 2001 Published by Elsevier Science B.V.