The site linked below reflects my understanding and experience; i.e., that your license will be accepted as long as it includes your photo and is in an accepted language.

http://idlicense.com/IDLNews/post/International-Drivers-License-South-Africa-and-Car-Rental.aspx

We rented through Avis at JNB in 2011 and didn't need an int'l drivers license. I would call (or email and bring the response with me) the Alamo office in CPT to double check.

From the South African government site:
http://www.southafrica.info/travel/advice/driving.htm#.UTIo8Wd62Sq

'Any valid driver's licence is accepted in South Africa, provided it bears the photograph and signature of the holder and is printed or authenticated in English.

However, vehicle hire companies may also require an international driver's licence. It is worth confirming requirements with your travel agent or the vehicle hire company when making your booking.

This holds for additional drivers as well, who must be identified when you hire your vehicle. Remember to carry all your documentation with you when you travel as traffic officers will expect to see it if they stop you for any reason'

We always have an International permit as we live in France, and our documentation is in French. but it appears that if your license is in English there shouldn't be any problem.

Of course if Alamo demands it ...

I have rented a car in ZA couple times without an international license. I could not see anywhere on the Alamo ZA site stating they require international license.

I lived in South Africa 3 years. I used my California license. Funniest experience was being stopped for speeding in a rural area and showing it to a Zulu officer who spoke little English. He looked at it and asked what it was. Then let me go. Another crisis averted.
Best advice I got from a resident is if you have a breakdown, leave the keys in the car and hide in the bush with your cell phone. Not worth risking a confrontation.

Kimwhits- you are making me nervous. I just called Alamo at Cape Town airport and spoke with someone from Europcar who evidently handles their rentals. They told me that an American drivers license will be enough. Thanks to all for your input

I am a little nervous about this trip - I will be meeting some people there but will be on my own quite a bit. I know I will be fine in the tourist area of Cape Town - staying near the waterfront. But will be driving to Hermanus and around that area solo - but from what I have read these are safe areas. I think I am just as worried about spacing off while driving on the left side of the road - any tips for that?