The asteroid 423 Diotima was observed photoelectrically in V filter during October 1987 at Observatorio Municipal de Mercedes. The synodic rotational period and amplitude obtained are: P=4.626 ± 0.002 hours and Δm=0.196 ± 0.014. The values confirm the solution proposed by Zappala et al. (1985) to the ambiguity presented in previous determinations of the rotational period and convert this asteroid in a peculiar object.