This paper presents a method for the joint estimation of the open quotient and the asymmetry quotient of the open phase of the glottal flow on speech. An algorithm based on a source/filter de- composition (the Zeros of the Z Transform - ZZT) is presented. This algorithm is first tested on a database of sustained vowels spoken at different voice qualities, then on running speech. Results are evaluated in comparison to the value of the open quotient obtained by analysis of the synchronous ElectroGlottoGraphic (EGG) signal. Results of this test show that open quotient is estimated within the just noticeable difference from the EGG reference in more than 60% of the cases, and that 75% of the estimations give a value within 25% of the reference. The estimation results on asymmetry are also discussed and confirm previous studies.