diff --git a/done.txt b/done.txt
index 5aec42131d86fe9d2b880cc447ea1460cd3589cd..f96403d7fa2afd47148c2e184f278ac478471639 100644
--- a/done.txt
+++ b/done.txt
@@ -7,3 +7,4 @@ x 2015-12-26 clean up tox files, make sure all tests run             @tests
 x 2016-01-03 check divfree function
 x 2016-01-03 compute kMeta(t) as well
 x 2016-01-03 split library into core and extra                                       @optimization +v1.0
+x 2016-01-07 FFTW interpolator doesn't need its own field                            @optimization +v1.0 +particle_api
diff --git a/todo.txt b/todo.txt
index f127ad3d4c61e9d987ead96b23b701a30d13b41a..8c38aeb973e9da0aabd488502b5acaf2d067aa57 100644
--- a/todo.txt
+++ b/todo.txt
@@ -1,4 +1,3 @@
-(B) FFTW interpolator doesn't need its own field                            @optimization +v1.0 +particle_api
 (B) add stat choice parameter to add_particles                              @design +v1.0 +particle_api
 (B) simplify tracer/field addition mechanism                                @design +v1.0 +particle_api
 (B) compute z polynomials only when needed                                  @optimization +v1.0