All Ph.D. students admitted to SEAS or EPS are guaranteed tuition and stipend, i.e., a graduate fellowship, for the normal duration of their time at Harvard. During the first academic year the funding comes from Harvard, after that it comes from our research grants. Students are encouraged once at Harvard to apply for external fellowships, because they give the student flexibility and helps bolster the laboratory research program. 

Here are links to two applicable external fellowships:

• National Science Foundation
• NASA