dissenting in part.

While I concur in affirming Taren-Palma’s conviction, I believe that the district court’s allowing expert testimony regarding Calderon-Perez’ use of a firearm during the commission of the offenses for which he was convicted is reversible error, and I dissent on that basis.

Clearly, in order to be admissible, expert testimony must not be unduly prejudicial under Federal Rule of Evidence 403. “A trial court should not routinely admit [expert testimony], but should carefully weigh the testimony’s probative value against its possible prejudicial effect.” United States v. Espino sa, 827 F.2d 604, 612 (9th Cir.1987), cert. denied, 485 U.S. 968 , 108 S.Ct. 1243 , 99 L.Ed.2d 441 (1988). Applying this principle, this court has held that expert testimony is only admissible to establish the modus oper- *537 andi of crimes in complex cases, and that in simple cases, such evidence is unduly prejudicial. See United States v. Lim, 984 F.2d 331, 334-35 (9th Cir.1993), cert. denied, — U.S. -, 113 S.Ct. 2944 , 124 L.Ed.2d 692 (1993); United States v. Lui, 941 F.2d 844, 848 (9th Cir.1991) (citing United States v. Johnson, 735 F.2d 1200, 1202 (9th Cir.1984)). While Lim and Lui were cases involving the admissibility of expert testimony regarding drug courier profiles, I disagree with the majority that the principles stated therein are less applicable to this case. As we indicated in Espinosa, the court must seriously consider the prejudicial effect of expert testimony in every instance, not just in drug courier eases. See Espinosa, 827 F.2d at 612 .

As in Lim and Lui , there was nothing complex about this conspiracy. It involved a simple drug transaction, a limited number of participants and occurred over the span of a few weeks. The jury surely was able to infer the connection between the firearm and the drug transaction without the aid of Kamin-ski’s testimony. Similarly, in United States v. Castillo, 924 F.2d 1227 (2d Cir.1991), the Second Circuit held that police testimony regarding the modus operandi of drug dealers, such as the connection between their possessing scales, baggies and firearms and their drug related activities, was unnecessary because jurors “are capable of comprehending the primary facts and of drawing correct conclusions from them as are witnesses possessed of special or peculiar training.... [W]e are not convinced that ... jurors ... need an expert to enlighten them as to such elementary issues.” Id. at 1232-33 . Because Kaminski’s testimony was equally unnecessary, it should have been excluded.

The potential effect of Kaminski’s testimony shows that its erroneous admission was not harmless. An evidentiary error is only harmless if it is more probable than not that the prejudice resulting from the error did not materially affect the verdict. See Lim, 984 F.2d at 335 (citations omitted). The evidence against Calderon-Perez was far less than overwhelming, and Kaminski’s ’ testimony transformed a possible inference from his carrying a firearm during the drug transaction into evidence itself. Most likely, this additional evidence doubly skewed the jury’s determination by also enabling the jury to disregard the possibility that Calderon-Perez’ possessing the firearm was consistent with his claim of innocence. 1 Furthermore, the possibility of such undue prejudice was greater in this case because Kaminski’s status as a law enforcement officer gave her expert testimony “an aura of special reliability and trustworthiness.” Espinosa, 827 F.2d at 613 (citations and internal quotations omitted). Thus, Calderon-Perez was impermissi-bly tried based on the standard techniques utilized by law enforcement officials in investigating criminal activity rather than the evidence against him. See Lui, 941 F.2d at 847 . For that reason, I believe that reversal is required.

I therefore respectfully dissent.

. Possessing a handgun is legal in Arizona. Moreover, while the majority states that the weapon was "concealed', loaded and cocked,” this was a disputed fact at trial obviously emboldened by the erroneous admission of Kaminski’s testimony.