An X-ray study of the effect of pressure and electric field on the incommensurate phase transition in ZrTiO4 was performed in order to obtain the information concerning the nature of the competing forces that give rise to a structural instability at a general wave-vector. It was found not only that an increase in pressure lowers the transition temperature (dT(i)/dP(i) congruent to - 6.9 K kbar(-1)), but also that an increase in d.c. electric field shifts the transition temperature (dT(i)/dE(i) congruent to + 1.7 deg cm kV(-1)) to a higher. The pressure and electric field dependence of the incommensurate to commensurate transition were also investigated (dT(c)/dP(c) congruent to -9.0 K kbar(-1)) and (dT(c)/dE(c) congruent to + 1.5 deg cm kV(-1)), respectively. (C) 1997 Elsevier Science Ltd.