A speech fragment from three Spanish-speaking adult brothers has been acoustically analyzed. The number of significant differences in the four first formant frequencies was calculated, per vowel and per formant. These differences are considered representative of interspeaker variation, while the intraspeaker variation would be determined by the significantly different mean values between two readings of the corpus by the same speaker. The results show that in F3 and F4, interspeaker significant differences are more often found, in comparison with the presence of intraspeaker significant differences. Nevertheless, according to the results for F1 and F2, these formants should probably not be discarded in Forensic Speaker Identification.