The Torah and the Gemara (especially in Yoma) gives us fascinating detail about the bigdei kahuna (clothes of the priests) that were worn in the Holy Temple. What is not clear is whether tefillin were worn by the kohanim when they performed their services. Do we have any sources that confirm whether or not kohanim indeed wore tefillin during their avodah?
The Talmud (Arachin 3b) informs us that the Kohanim were exempt from wearing the Tefillah shel Yad while servicing in the Temple because it would constitute a separation between the priestly garments and the skin. The Tefillah shel Rosh could still be worn, and the priestly garments on the head were worn in such a way as to leave room in the front for the Tefillah shel Rosh. However, the Talmud (Zevachim 19a) also tells us that Kohanim, Leviyim and Yisraelim who were serving in the Temple were exempt from wearing Tefillin, as Rashi there explains, because of the principle עוסק במצווה פטור מן המצווה one who is busy with one Mitzva is exempt from another. Rambam (Klei Hamikdash 10:6) says that the Kohanim could wear the Tefillah shel Rosh during the Temple service if they wanted to.
2Woohoo! I was Mechaven to Zevahim! Seriously, though, what about the fact that, since they don the Tefillin in the morning, it would come first? Hence, perhaps, they might not be Peturim? Also, I seem to recall discussions about not doing Mitzvoth from which one is Patur because of 'Osek BeMitzvah, as it causes a distraction or a Hefsek. Is that not considered by the RaMBa"M, or is this not considered to be a problem in those regards due to timing or practical application?– Seth JJun 25, 2013 at 14:34
@SethJ I'm putting Rashi and Rambam together here and assuming the two Gemaras don't argue. Your questions are good and I'm guessing there is more to this sugya. Let me know if you find anything and the same for me.– Double AA ♦Jun 25, 2013 at 15:56
I doubt I'll remember to look into this later, unfortunately. Lots going on today besides fasting. (Also, even though it was still morning, I am blaming any incoherence on the fasting. That's my story, and I'm sticking with it.)– Seth JJun 25, 2013 at 16:50
2@SethJ See Beis HaLevi 1:3:2 (about half way through) who argues that Rambam doesn't think Oseik beMitzva is at play here, and see Zechor leAvraham 1:64b who tries to answer why Oseik beMitzva might still result in a Reshut status here.– Double AA ♦Jun 25, 2013 at 19:13
1@double-aa Based on your suggestion I did a little web search and found: "The gemara (Zevachim 19a) establishes that the tefillin of the kohen gadol was to be placed between the tzitz and mitznefet." Also, it turned up BT Arachin 3b which asks the question I asked, is there room on his forehead.– Yehuda WApr 15, 2015 at 13:44
רש"י (שמות כ"ח ל"ז) describes the placement of the straps on the ציץ that was worn by the כהן גדול to be consistent with wearing תפילין at the same time.
על פתיל תכלת. ובמקום אחר הוא אומר ויתנו עליו פתיל תכלת? ועוד כתיב כאן והיה על המצנפת, ולמטה הוא אומר והיה על מצח אהרן. ובשחיטת קדשים שנינו שערו היה נראה בין ציץ למצנפת ששם מניח תפלין
I recall that the tefillin would constitute another beged and they would be over Baal Tosef? Likewise, even though some of the begadim, the Ephod, the Mitznefes, the Avnet, had 4 corners, they had no Tzitzis for the same reason.