The results of dislocation density and 90degrees-twins investigations in [1 1 1]-oriented InP crystals grown by the LEC technique with two different axial temperature gradients, dT/dz = 260 and 100 K/cm, are reported. Estimation of the maximum diameters (D-max) of dislocation-free crystals, which can be grown with a given dT/dz, is made. It is discovered that 90degrees-twin formation for a given dT/dz takes place in a narrow range of crystal diameters (D-T), and D-T approximate to D-max for both dT/dz = 260 and 100 K/cm. By the selective etching of the crystal axial section, crystallization front shape in the 90degrees-twin region is revealed. It is discovered that 90degrees-twin formation coincides with the morphological signs of a change from crystal dislocation-free growth mechanism to a dislocation growth mechanism. A possible mechanism of influence of the dislocations on twin formation in the framework of the theory of the crystals lateral growth from the melt is discussed. (C) 2002 Published by Elsevier Science B.V.