We present multicolour photometry of four-field ultracool dwarfs with the triple-beam photometer ULTRACAM. Data were obtained simultaneously in the Sloan-g' band and a specially designed narrow-band NaI filter. The previously reported 1.8-h period of Kelu-1 is here recovered in the g' band, but the lack of any significant variability in the NaI light of this object precludes any conclusion as to the cause of the variability. 2MASS 2057-0252 and DENIS 1441-0945 show no convincing evidence for variability. 2MASS 1300+1912, on the other hand, shows good evidence for gradual trends in both bands at the 5 per cent level. These trends are anticorrelated at a high level of significance, a result, which is incompatible with models of star-spot induced variability. It would seem likely that dust-cloud `weather' is responsible for the short-term variability in this object. Based on observations made at the European Southern Observatory, Paranal, Chile (ESO programme 075.C-0505). E-mail: s.littlefair [at] shef.ac.uk (s[dot]littlefair[at]shef[dot]ac[dot]uk)