How to select rows with values set to NULL

Hello,

InfluenzaWeb=# create table test (
InfluenzaWeb(#   id int,
InfluenzaWeb(#   txt varchar(17) default NULL);
CREATE
InfluenzaWeb=# insert into test values( 1 );
INSERT 553756 1
InfluenzaWeb=# select * from test;
 id | txt
----+-----
  1 |
(1 row)

InfluenzaWeb=# select * from test where txt = NULL;
 id | txt
----+-----
(0 rows)

InfluenzaWeb=#  insert into test values(2,'text');
INSERT 553757 1
InfluenzaWeb=# select * from test where txt like '%';
 id | txt
----+------
  2 | text
(1 row)

InfluenzaWeb=# select * from test where not txt like '%';
 id | txt
----+-----
(0 rows)

So how to get all rows which txt has the value NULL?

Kind regards

         Andreas.