We plan to book tickets for the ballet "Romeo & Juliet" at the Prague National Theatre in October and need to know what the dress code is for a performance. Not planning on taking dressy clothing and really hope slacks instead of jeans might be permissible. Also, I assume I am safe booking through www.ticketsonline.cz ?

Smart casual should be ok but don't be suprised to see many people dresed in suits and smart evening wear.
i have never used ticketsonline but usually use
http://www.ticketsbti.cz/WBS/ang/
when I am in the city for any shows,never had a problem with them.

Thank you so much unclegus for the info. I went to the site you suggested and have purchased the tickets. It is a little cheaper than ticketsonline and the service charge is less. I have the e-tickets printed, and appreciate your recommendation. On the tickets it tells me just like you did to dress smartly, but jacket & tie not necessary.