A method for observing biological species on a culture medium contained in a container having at least one translucent face, the method including the steps of: a) directing a light beam onto one portion of the translucent face, so as to define at least one illuminated region and at least one non-illuminated region of the face; and b) acquiring an image of a portion of the surface of the culture medium illuminated by the light beam, the acquisition being carried out through at least one of the non-illuminated regions of the translucent face and along an optical acquisition axis forming a non-zero angle (a) with the direction of propagation of the light beam.