Observation of the phase front and the domains during the paraelectric-ferroelectric KD₂PO₄ phase transition has been performed under small dc applied electric fields E_z. The dielectric ε'_c and loss ε''_c constants have been simultaneously measured. A special phenomenon during heating cycles is observed with creation of a heterophase region with alternating paraelectric and polydomain ferroelectric stripes in a sample which is essentially monodomain. This is explained with the help of the mechanical energy calculation due to the inclusion. The increasing of the thermal hysteresis at the transition when a small dc electric field (or stress) is applied to the crystal is also explained.